I did something really low and I'm super ashamed of myself

I've been staying at my aunt's house for a visit and she has an 18 year old daughter (my cousin) who's recently started college. I always thought of her as a sister when growing up with her even though sometimes people including my own mom would joke about us getting married.

Yesterday my aunt was working late at night as she's a single mother so it was just me and my cousin. She said she might invite a friend over so I was like ok cool andI went back to stay in my room and play games. I hear the door open eventually and I hear a guy's voice. I didn't know she was the type of girl to have guy friends. She doesn't wear hijab but I always thought she was innocent and didn't engage in this haram stuff westerns always do. First I hear then downstairs laughing and chatting among themselves. I even went down to get some water and jokingly asked her if he's her bf but she introduced him as "just a friend" and said he already has a girlfriend (he even showed me his phone and it was a pic of him and his gf on the lock screen). I thought nothing much of it and went upstairs again and minded my own business. Eventually I had to go use the washroom so I went towards it and also saw my cousin going in before me. I asked if I can go first since I drank too much and had to pee but she seemed very hesitant to let me go first and wouldn't even let me enter through bathroom or even look inside and told me to use the downstairs one so I did. Eventually as I'm coming upstairs I hear whispers in the bathroom. I stay close to it to keep listening and it seems like she and that guy were both in the bathroom together. I felt like throwing up like how could she be like this? I always thought she was a good muslimah who prayed on time, fasted and now I see her do this in the bathroom with a non mehram. Eventually I start hearing her moan. I was shocked and didn't know how to react but I did feel sad and disappointed with her. Then I did something really shameful and started masturbating myself to the sounds of her moans. I don't know why I got that urge. I felt saddened and disgusted and disappointed by her but I couldn't help but find her moans attractive and I came listening to a guy doing things to make her moan. I went back to the downstairs bathroom quickly after I came to clean myself and by that time they had both come out. She was downstairs when I came out and assumed I had been in the bathroom this whole time asking me what I had been doing in there for so long. I went back to my room after that and didn't come out. I don't even know what time the guy left as I fell asleep shortly after.

Even in the morning I felt disgusted by myself and can't even look at myself in the mirror after how low I stopped. I haven't told my aunt anything yet and I don't think I can. Her innocent heart won't be able to handle it if she realizes this is what her daughter is up to. I don't think I wanna stay in this house any longer and I don't know what to do. What would be the right thing to do as a good cousin and good nephew? Even I did something wrong last night so should I really be exposing her for what she did in secret while my secret remains safe? So many thoughts in my mind and idk what to do and its making me depressed.