I am slightly puzzled by some of these responses.
AThingInYourLife said: "...the evil ex who took her legal due, so she allowed this man to financially fuck her over?...He knows she is vulnerable financially because he has put her there to suit himself."
Strictly, as has been observed, she put herself there; and he's taking his legal due, too. What's the difference, exactly, between his position now and his ex-wife's when they divorced?
Then Feckbox wrote: "I owned my home before we got together. He has seldom worked. We are not married . THANK GOD "
Presumably Feckbox is thanking God she's not married for the same reason the OP's partner wishes he hadn't been: unmarried, whoever brought the money keeps it.
VivienneMary wrote: ?If she has contributed to the house then she is entitled to have her name on the title deeds.? Should she take on just the asset, or the debt as well? Who should pay the gift tax involved?
Lois wrote: ?Never, ever have a child unless you're married.? I agree, but supposing marriage isn?t on offer; what do you do then? Because this seems to me to be where we are moving to.
?If he wanted to do the right thing, he would.? How has that worked out for him up until now?
But rather than arguing the toss about how much of a bastard he is, what are the two or three very best, most persuasive arguments for why he should marry the OP?s sister?
?I?ll leave if you don?t? is actually a very good argument for him not to marry her. If someone threatens you from a position of weakness you'd have to be mad to place them into a position of greater strength.
Likewise, "marry me because it will make me happy, which you should want" can of course simply be turned around, and then you have a Mexican standoff.
It's clear that wills need to be drawn up, but the form his should take is not simple given the various dependents, so it's not like there's something obvious and easy he should do but won't.