I am sorry for what you are feeling. It really can feel so overwhelming at times. But ask yourself this, what really has changed? Okay, you have some further evidence that your FIL is a nasty controlling guy. As for what will become of your R with B and hers with your son or her parents' relationship with your son, only time will tell.
You have enough on your plate with which to cope, try really hard to put this news out of your mind. Easier said than done, I know. (H is in Belgium with his folks and it is consuming too much of my free thought time, wondering what they might be telling him to do.)
As for losing people who have been family for 10 years. I know exactly how you feel. The same thing is happening to me. I guess all we can do is to tell ourselves the outcome of those relationships is on hold for a while.
I am not a big online hugger (as Tawnya will tell you), but I feel like you need some hugs tonight so here goes...{{{{Carlos}}}}