He can't cope with any of them other than at extreme arms length, so we are LC and ask for nothing.
I'd keep it that way. If his family are as bad you're saying to the point they affect your husbands mental health the way it does, I'd keep them at a distance like that.
I say this as someone whose father fully funded and supported my drug addict step brother through all sorts of stuff my brother and I could only dream of while leaving his biological children in poverty. My dad died leaving a lot of money to his stepson and my stepmum, of course five years later it's all gone up his nose and he's on benefits and stepmams money has probably gone up his nose too.
I'm no contact with the whole of my dads side of the family since my Dad died five years and it's the best decision I've ever made. They've got no power over me at all now and can't hurt me and that's priceless. I'd never go ask stepmam for money to fund my child through uni, if myself and her father can't do it then loans and a job it is. DD herself won't have any contact with them and the first year after I went no contact with them I made it clear I'd support dd in maintain contact and dh would have facilitated it and that option is and always will be open to her, but both dh and dd said no and they'd have ditched them long before I did as they saw the anxiety and stress they put on me with even just low contact. Can't lie, it felt good to know they supported me but if dd said tomorrow she wanted contact with them I'd support that and not make her feel like she can't.
Stepmam actually sent a cheque for dd of £1,000 a year after I went no contact with them, it was inside a bday card, dd tore it up saying it's a trap so she can say she only wants her money.
My brother also wants nothing to do with them but did the same with sil and the kids and said he wouldn't expect them to stop contact, sil is very bitter about not getting the same financial support as step brother and is often telling him he should be "getting his share" she stays in contact with them and I think it does hurt my brother a bit as sil will repay all their convos back to him and it brings some horrible memories for him and his own mental health suffers and seems to take a dip when she's been to visit them.
I think you need ask yourself if any amount of money is worth them holding this over your husbands head. Are they the type who'd be "we funded your son through uni and you can't even visit"
What will having to thank his parents for giving his son a generous amount of money do to his mental health given that he doesn't want to even ask them?
Instead of being bitter about the money they've given to others, feel proud and happy that your husband turned out differently and has a good settled life. That despite being raised by the same parents he's turned out so different to his brother, that he wasn't so fucked up to have had sex with a child and get her oregy, that your son is lucky to have parents who've raised him in a stable environment and is a functioning adult who can take a job to support himself through uni as others do. I wouldn't be bitter and jealous of your sons cousin as it sounds like he had a bad start and was born to child and had a deadbeat dad.
I agree it's not fair but like they say, life isn't fair and if you really do have your mind set on asking I think it maybe would be better for your son to ask them himself and not frame the request in a - you have cousin £50k so you have to give me the same, but in a "please could you help me" way.
Best of luck to all of you.