Opinions??

How would you feel if your dad is with someone that is your stepsisters cousin? She’s like 40 and my stepsisters are like 27 but I think that’s disrespectful. Rumors were they were kissing etc. I didn’t believe it until I saw it with my own eyes. I mean they kissed right in front of me! My dad would sneak out the house at 12 and come back at 2am. I honestly don’t know how to feel about the whole situation given that my mom and dad are still married but are separated. Am I overreacting? Is this normal ? I am right for feeling this way? I confronted him about the situation but he always changes the subject, doesn’t answer my questions or gets mad. It’s non of my business what he does with his love life. I’m 17 btw but I feel disrespected in a way. He doesn’t own up to what he does which is the least he could do. If he’s going to do something might as well come clean there’s no reason to hide have some dignity for fuck sake