I think it turns me on that my mom had an affair and my sister slept with a guy. Is that a strange thing?
Would this same thing turn you off if you heard about it happening with your dad?
No, I don't think so.  It's just a matter of interpretation.
It's wrong to be turned on by family members' sex lives.  In general, any sexual activity they have is their own private business, and you should not be trying to find out about it, even if it is appealing to you.  They wouldn't want you to know or to speculate about these things.
I don't really care what they want. I just like getting my rocks off.
Family is there to support and love you but not to support your sexual desires. Find another muse for your sexual outlet and stimulation that isn't familial.