should I do nothing?

Having worked in domestic abuse I would say please keep doors open. Your niece may need someone to be there for her one day.
It doesn't sound like a good start though does it...
A close family member was involved with a controlling man and he did everything he could to cut family out.
So if you currently have any contact with your niece keep it going if you can. If you don't usually contact her, you can always pick it up at the wedding by saying "how lovely to make proper contact again". Send jolly texts ( but not only texts as phone may be controlled), postcards from holiday to them both maybe, anything that says subconsciously that you are with her.
Perhaps this will all be unnecessary (I hope so) but at least you will know that you haven't just ignored your concerns.

I have faced this in the past with my daughter, once it was a dreadful controlling boyfriend and the police were involved at the finish of it. The second time when she married someone who just wasn’t right. It ended in divorce. I didn’t interfere in either case simply because I was terrified of losing her. I married a man who had been divorced and his first marriage had taken place behind his family’s back because his father had told him he shoukdn’t Marry her and the family didn’t like her. MOnica is absolutely right, these marriages do take place, sadly. My DD was divorced. What we all did was to make very sure she knew we all loved her and never, ever said “we told you so” when it was all over. It was so hard to watch. She’s married again to an old boyfriend and they’re so well suited and happy but she can’t have children. But that’s another story’s,

Do nothing for now. Keep communication open with your neice, so that you can be there if needed at a later date.

If there comes a day when she does need help, she is unlikely to turn to people who might say 'I told you so' (even if they wouldn't).
Least said soonest mended.
Be wary of communication channels being closed down and keep in touch even if you don't get much response.

.....and stop squirrelling away together to discuss her choice of partner or her difficulties real or imagined, behind her back
There is close and there is intrusive

CatLover123.Only you know your niece and view her in your own words as ' vulnerable'.
This then rules out independence /decision making characteristics frequently observed in many 21C women.
From your description of her boyfriends past and present I have the impression this' vulnerability' is picked up in the early stages of her relationships hence the way she is being treated seen by others as unacceptable ,which it is, and not the basis of a lasting relationship.
Stay away until invited is my advice.

If I have read your post correctly this young woman has a history of being in relationships with controlling men.

If that is the case she will only change when she has had enough and gets some help to make that change.

As others have said "stay out of this situation". This young woman is making her choice. If she has the better job and the support of her family she has the option to get out of this relationship. She isn't making that choice.

Again others have made the point that nothing anyone can say will stop these types of processes or relationships going forward.

It is sad, it is difficult for those around to watch a loved one going headlong into hardship and pain but it is their choice not ours so watch, wait and hope we must.

Sounds like there are plenty of other family members concerned for this young woman, relax and let her follow her course.

When did any of us ever take advice re relationships? When did anyone we have given advice to heed our warning?
Probably never ....... until we learn it by our own mistakes we never, ever learn.

Nothing you can do.

I would do my best to continue to have a good relationship with my neice by keeping in contact etc and if possible say you are always there for her if she needs someone to talk to
and leave it at that.

Difficult but family come 1st. I would speak to her mum, it is everyone's business to keep family members safe & cared for, i don't understand why we should keep quiet here, yes there may be fall out but that's family life, everyone has her best interests at heart which is paramount. Not speaking up let's him carry on.

Catlover123. I'm so sorry! For you and this poor girl who clearly doesn't value herself highly enough or maybe is scared that she is getting older now and has to take whomever she can to marry her.

I can feel how this is nagging and upsetting you. I am sorry to say that should you try to do anything it could make things worse. It is better in my humble op. to keep your counsel. I know this is hard and you want to protect this girl/woman but I think by disclosing this private message you could start a family rift which would help nobody. You all need to be closely united in case the future holds unhappiness for your niece.
Men can be very sensitive about their job and its standing. I have witnessed this many times. I am unhappy that the girl has to keep quiet about her job and it doesn't bode well for her happiness in this relationship. You may best be of help by simply showing your love and support to her and her mum, so they know they can talk to you and lean on you if needs be. Could you pay a visit, just naturally because you like to see her?

theretheredear- why do you think the OP speaking to her nieces mother will do anything at all to stop the partner from 'carrying on'.

Family does come first but are you suggesting that the other people saying exercise caution are not putting family first?

I wouldn't express your doubts to anyone in the family. If your sister-in-law, who I assume is your niece's mother, admits she has doubts about her prospective son-in-law, you can admit that you don't quite like what you hear either, but don't bring it up unless she does.

Otherwise put a good face on it, because any criticism that you or others express to your niece will be badly taken.

I would take the engagement as an opportunity to have a talk with your SIL, in between asking about the ring and possible wedding dates you can ask about his family and so on, surely this is a conversation you are going to have anyway. During the course of the chat you will be able to work out if your SIL sees any problems, if not keep quiet. Might she have rung you if she had concerns? None of you can do much about a 30 something determined to get married.

Don't say anything, not your business.

It could be that your niece's parents or other family members have noticed his manner/behaviour if it has been obvious to your DiL. Let them sort it out. NOT YOU!

Playing devil’s advocate here, but I think your niece’s previous history of controlling boyfriends may be clouding your judgement. It could be that your niece was unwilling to discuss her new job out of her own respect and deference to her fiance, and not something he was aware of. Just a thought...

Hahaha....in her 30s and you're worried about her b/f? She's not even immediate family. Good grief.
Why on earth do you think that it's anyone else's business? Keep out and MYOB.

Im a bit different and it might be because my culture will not allow my child as it would be termed even though its your nieces
To step where there is danger. If there are concerns I would have too say something ..its how I say it thats key ..wont allow my own to step in the lions den without raising an alarm.
Never could understand this keep out of it advice, why would you not show concern if there is something to be concerned about

Any criticism of family's friends/partners etc. rarely ends well. I'm afraid, you have to let people make their own mistakes and just be there to pick up the pieces if things go wrong and try very hard not to say, " I told you so". It's difficult to see a person you care about with someone considered unsuitable, I know, but unless you are asked outright for an opinion by your niece, it's best not to say anything to her personally.

I agree totally with your comments quizqueen

don't get involved .but be there if she needs you .

Dont go there!

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Difficult to even think of anything when you are hearing everything second hand!
I think you need to meet the man before forming your own opinion !
In this day and age, personally, never heard of a woman feeling her partners masculinity would be affected because she has a better job! Anyway, she may well marry him anyway, whatever anyone thinks or says. I'd keep out of it!

Depends how close you are to your niece. Do you know if she would welcome a friendly talking it over with you?