Trusting future SiL

A clear case of coercive control.

However, what does this mean?

“There is a history of abuse with her sister in her early teens & im scared there may be an untapped backlash,”

Who was abused? The OP’s DD? The sister? Did the DH abuse one of the girls or did one of them abuse the other? Where is the backlash coming from?

Until we know for sure, it is difficult to see how this fits in with the picture.

However, the DD should run for the hills. The OP should alert every interested official body she can to protect her DD.

Firstly - I never understand why people apologise for a "rant". I remember when people just spoke as they felt and the word RANT was never mentioned. However, as for your concerns.... you will realise that this is a control matter and is not acceptable. Destined for the rocks, eventually. The common experience will show that "interfering" is not accepted and often shunned. You just keep in contact and keep your eye on the ball. Personally if I saw serious control I would put him in his place and let him know my level of tolerance. But there is nothing you can directly do which will magically change things. If I tried to stop my wife from drinking she would tell me to get stuffed, in no uncertain terms (i dont joke about the situation) but some people are not so strong of character and it is this weakness which allows for the controlling partner to be in control. I would pack my bags asap if for no other reason but to assert my own being as a person, an individual.

Your are so right to be concerned. This is coersive control, classed as domestic abuse due to the mental control and is as bad if not worse than physical abuse, you can't see the bruises! Your daughter will say she is alright out of fear of any consequences. She probably feels trapped and rock bottom. All you can do is be there for her, the more you say the more she may shrink into the mental prison he has created. Sorry to sound hard but I have been there!

Johno if the OP were to speak out to "put him in his place" she could find herself being branded a trouble maker and maybe cut out of her daughter's life.

Frankly I am gobsmacked how anyone can talk about abuse having so little evidence (2points) and they are absolutely nothing to go by.
DD has given up her car but the OP can’t go into the details of why. Nowhere does it say he made her give up the car. She may not want to drive anymore (maybe following an accident for example)
He abhors alcohol - so do I. If she drinks in secret she could be an alcoholic. For all we know the OP and family could be alcoholics and not see anything wrong with it.
It is impossible to give any advise to the OP other than support DD without interfering.

I totally agree lumerei the reason I haven’t commented
If the poster comes back with a bit more info it might help

there was a history of abuse with her sister in her teens what does that mean ? Was the sister abused by a boyfriend and the poster is extra vigilant now because of that?

As another perspective, I know a couple where she hides her drinking and spending, she has lost her car & they have moved miles from her close family.....

..... the truth is that she keeps running up tens of thousands in debts. They lost their second car and house for that reason (now rent in a cheaper area). She has previously seen people for her spending addictions and debts and agreed spending/payment plans with professionals but then secretly spends or sneaks out for £200 of cocktails.

I love her as a friends but I dont know why her DH stays with her. I think its because his family isnt very warm and she's all the family he's got! I think her lying and running up debt puts him in an abusive relationship. He keeps believing her each time they agree a plan and works his butt off to pay off her debts whilst she runs up new secret ones..

I only know this because I know her very well.

Red flags are important. They are a sign of a problem. And my friend does have serious problems and needs help (and needs her family to stop enabling her spending/drinking when they are losing their home and car and up to their eyes in debt but thats another rant..)

Dont ignore red flags. But keep an open mind as to what is causing the flags so you can offer the right support