HELP!!! I'm confused

So look this is something that's really complicated. My dad and someone who I considered as a sister are now having...relations. She has kids and I'm assuming will be going through a divorce soon and he's older by 10+ years. Obviously age doesn't mean anything but we all stay in the same house fam. It's been going on fora few weeks as far as flirtation back and forth then last night these people just took it all the way and I was asleep as were the other spawn. She told me this morning. We have a after close relationship but now it's starting to eat at me but they're grown so I can't really say anything because adults do what, and apparently whom, they want. Not completely sure how I feel yet especially knowing it's not over. I am at this point: irritated, disappointed, disgusted, confused...that's all I've figured out so far. These people talking about laying in bed on a nightly basis. I even tell this story the whole way but my mans predicted it. He's always on point with things that he says is going on. How am I supposed to feel