Split between family and partner

You may not want to end your relationship with your fiance, but I definitely see some red flags here!
I think you both need to take a few steps back, if you don't want to cancel the wedding at least put it on hold until your lives are more sorted. Do you expect your daughter & her family to move into their own place any time in the near future? Does he understand that you'll still have a child living with you and, although not living in your house your daughter will still need you at times?
Who will look after his disabled parent if/when he moves out, presumably to yours? Are his parents living near to you, so he could still help?
Personally, I'm wondering if he's hoping that by marrying you he will get out of caring for his parent/s, or that you'll take over? But for you to take over his responsibility, then your 'responsibilities' have to go!
I think, before you go ahead with the marriage, before even going further in your relationship, you need to sit him down, have a chat, find out what he's expecting and put forth to him that your daughter is a huge part of your life and you won't have her, or her family, pushed out!!
And also make sure he knows it won't be up to you to take on his family!
Good luck!!

He certainly does not have your best interests in mind the best thing is to get rid of him or you will end up caring for his parents as well as your own family.Show him the door asap.

This is really difficult for you BUT I totally agree with all the other posters
Do not get married !!!
First you ve never lived together so no idea if your habits will suit one other, second, he has responsibilities with his parents you with your daughter and grandkids so how are those two big differences going to miraculously disappear
What are your plans? you say you can’t both live at yours because its too crowded, you can’t live at his because of his parents, so what were the plans ??

I could NEVER put a man over my children or grandchildren who needed me particularly a man who didn’t seem to understand

Don't get married.!!

You sound very stressed and this is without doubt a complicated situation you find yourself in and most people would struggle.
I would make it clear to your partner that your situation will not change if and when you marry him and he needs to come to terms with this and listen to what he says.
If he just expects you to walk away from your children at their time of need he is probably not the person for you.

You have adult children and GC why consider a man who sounds like a child and he needs a dummy. The situation is not right do not marry him and get your priorities right stop letting your family dictate your life. You love them of course you do but don’t let them rule your life get some space and some you time.

You are neither of you fully committed to this marriage.
Your priority is your adult daughter, her two children and her partner, who appear to have taken over your house.
Your partner lives at home with responsibility for a disabled parent but no experience of children.
Both of you need to compromise, but it seems unlikely to happen. You would be very foolish to go ahead with this marriage at present.

He sounds like a child not an adult.

Alarm bells are deafening. Don’t even think of getting married.

I’m sorry to say this but your partner seems to think he should be the centre of your World and doesn’t understand that you have a family to consider.
Please take a massive step back, put the wedding on hold and if he loves you he will wait but if he doesn’t, personally I think you will have dodged a bullet.

I agree welbeck.
From what you have said it does not bode well!!! Marriage - please think again , too many issues. Your daughter needs you and your partner should understand and support you. Be very careful!!

do not get married.
frying pan, fire.

Hmmmm, but he is clearly equally committed to his family? Or does he want you to help with his family? Hmmmm, Do you have holidays away together? I would certainly be putting my wedding on hold as, simply reading your post, it feels like frustration is building, on both sides and perhaps a breather is called for. Maybe neither of you are in the right place, given your family commitments to make any further commitments right now x

He can be very supportive if the baby is ill and needs to go to hospital, but isn't supportive of my daughters mental health needs. Everyone thinks because she is an adult she should just cope on her own, but mental health isn't that simple and she is my daughter, no matter what her age. He has his own mental health problems but he fails to understand hers.

If a partner tried to tell me not to support my children, he wouldn't be my partner for long. Why on earth are you contemplating marriage to someone you haven't lived with? And who is apparently so unsympathetic to you and your family? Walk away. He really doesn't have your interests at heart