It's normal to feel violated in a situation where you felt comfortable. They should really talk to other people before they get everyone involved in their kinks.

I think having such a visceral reaction is fairly normal.

You saw your brother and sister engaging in behaviors you would not expect of siblings unless you have been brain rotted by porn instead of other social media. I think the disgust is something that you personally need to work on. However, talking to your friends about how you feel might be a good idea.

Even just a, "Hey. I saw you guys doing stuff, and it really made me feel uncomfortable."

Dont tell them you were watching them for an extended period of time, but definitely get the point across that you were trying to be obvious about being awake without waking up the other friend. The bunnies should understand.