Or, more likely, they're both happy, fucking more than you and sharing a chuckle at the loser of a brother.
You don't understand those kind of women, they do that kind of thing for the thrill and that thrill cannot be found just by having intercourse with their partner or get substituted with her partner love.
So if they married, she will do something behind her husband back. At best she will have some kind of OF account which enough to satisfy her need for thrill, at worse they will have a 'happy marriage' until the brother is 70 y.o. and found out all of his children aren't his and he died from stroke after knowing all the truth.
I will said it again, If you find your woman is that kind of woman, you better stay away from her. If you can tolerate her act, then do whatever you want with your life.