Look, she is only 28 and already has three kids and two divorces under her belt. You need to get away from her as fast as you can! There is no future with her.
You can't bust a divorce when there wasn't a marriage to begin with. And, if her feelings has turned this cold in four months.....you can expect it to be like this from now on. Don't stay with this woman. Don't take responsibility for her, her three kids, and her sister. You could meet a girl who really loves you and wants you, and who doesn't have a house full already.
Take my word for it, she'll have another man by next month.....if she doesn't already. Please don't get tied up with somebody like this.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!