Thank you for this piece of advice. I didn't think of that. So this problem might be solved. Now I need to find the right time (my brother is 8 years older than me, so we don't see each other all of the time, even though we still live together). I'm a bit scared of having to explain everything all over again (I had to explain a lot when I came out to my mom, and she had a hard time processing it. She's supportive now 😀), but I guess that's the price I'll have to pay.
Now I just have to find a way to deal with my cousin and my other cousin's boyfriend.
Thank you!