Breath...just breath. Wait until you get the results to panic any more, but then only allow yourself a few minutes to do so because it is what it is and you will be given the tools you need to get through it. Panicking will only make things harder then they need to be.
I understand your fears. My best friend is in a similar situation with her live in boyfriend and the girl he cheated on her with. She's no where near poly minded and needless to say her and the teenage baby momma don't get along at all! Yet, she and her boyfriend of only a year are coping rather well with everything. They have even found a way to co-parent this child with the mother. I tell you this story because it sounds like what you have with your relationships is much stronger then what she has and she's still making it so that means you can too. It sounds to me that you would be a lot more needed by both of them then you seem to think. I'm sure once they see how much you're willing to be there for them they will be that much more appreciative of you and your needs that will come along with this. Good luck.