Was I wrong for telling my sister she disrespected me when she had sex with him?

Anonymous
This is really long but if you could take the time to read it and help me out I would appreciate it.

My sister's boyfriend is our cousin (yes, that is completely disgusting but I'm sure some people won't find anything wrong with it). Anyway, she told my parents a while back and they don't agree with it and everything fell apart. My parents no longer think she is dating my cousin but she is and I have been covering with her for all these months, and I hate myself for that. But apparently he treats her so well and he's a good guy (they are both good people) which is why she wants to stay with him because she says they aren't many good guys left anymore. I'm still uncomfortable with it though and I feel like I've lost my best friend.

Anyway, today he came round when my parents were at work and I was going out shortly afterward so I told them I went to jump in the shower. But I didn't jump in the shower immediately because I was waiting for a film to download on my laptop. afterward when I went up to my room they were having sex and it was loud. I could hear them moaning and the bed squeaking and bouncing and everything. I already knew they've been having sex but I never would have thought they would have sex in our house when I was there. They know I'm really uncomfortable with their relationship already, and them having sex while I was at home just pushed me over the edge. They went too far. Especially after I have been covering for them all this time. It's like they are taking advantage of my good nature and support. There's certain boundaries you don't cross. I could easily tell my parents but I haven't because it would destroy their happiness.

Before I left the house I said to my cousin "You could of at least waited until I left to strt doing what you were doing!". He replied by saying "W-w-w-what do you mean?" and I replied "Don't give me that S***!" and he just laughed nervously and I walked out the house.

When I came back my sister confronted me and said "You didn't have to say that to him!" and I said "You should have waited for me to leave the house before you have sex with him! You already know how uncomfortable I am with you and him being together!". She then said in the harshest tone "I don't have to wait until you leave the house! God, you're acting like you don't know what couples do in relationships". To which I replied "I know perfectly well what couples do. This isn't about you 2 being a couple. This is about being disrespectful towards me by having sex with him when I am still in the house. I have covered for you for such a long time and you end up not giving a f*** and taking advantage of the situation!". And then just walked away.

What do you think? Am I in the wrong here?
Was I wrong for telling my sister she disrespected me when she had sex with him?
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