Being one who has cheated on a spouse and had an affair with a married man, I would just back off till she gets things figured out. Some thing that you can point out to her is that she can get her divorce if she should choose to go ahead with that and still have the father in her kids' lives. One thing that I have always seen with parents who stay together for the kids' sake is that the relationship still doesn't work and all that happens is it makes everyone miserable. Including the kids.
The only person who can decide if you are getting into more than you can handle is you. But I would, if at the very least, is give her room to decide what she wants and where she wants her life to go. Who knows maybe she can find away to have the kids in their dad's life and still find the happiness that she is obviously looking for without making everyone miserable.