Wow. This thread makes terrifying reading. And sorry, OP, YABVVVVVU. You're sister has been royally shafted (as so many women are) and not only have you been brainwashed into thinking her ex is a charmer, you think SHE is unreasonable?!
They were together 20+ years. If she had a little A4 piece of paper (that a few PPs on here need to remember TWO parties have to agree to get) she would be entitled to half of everything. The reason for that entitlement is she didn't work - presumably at her ex's agreement - is because she raised their children. Or is that worth nothing on Mumsnet?
Now they've split - irrelevant why. And he's been OHSOVERYGENEROUS 'allowing' her to stay in the family home until the kids are 18, but after that - "thanks for the years you sacrificed to raising my children, sorry we never got round to getting married, no off you fuck I have a new woman I want to move in".
Soooooo interesting he never pushed for marriage when he bought the home before her, isn't it? Almost as if he didn't ever want to share it....
I do think your sister is being foolish not to work, pretty inexcusable to be honest as the kids are grown. But you need to show some loyalty here.
He has absolutely shafted your sister. The fact your parents are inviting him to family events is weird. If they have to pick a team it should be their daughter.
And I'd be having serious words with my DH for telling him he's being 'a mug' to make a morally better decision that UK law, given he's already been morally fucking bankrupt for not splitting everything 50:50 after two decades.
And, FYI, maybe tell your sister to speak to a solicitor. In the UK if you raise a family in a home, aren't married and one person owned it before, you may be entitled to a share.