At this point I really don't get MC.
She gets his wealth and save her sister, in the novel he gets with the "heroine" (or whatever) because the sister died and he was super fucking sad.
He's not married to the sister, she won't die (like in the novel at least) an has no reason to get with the "heroine"
Worst comes to worst, he still gets with the "heroine" but break/divorce MC, but what's the problem with that? She doesn't like him, and she already has a bad reputation so it's not like it's gonna ruin her much more, and there's even the possibility he would offer her a lot of wealth for a divorce.
I literally can't see a single downside to the current situation.