My best friend(M15) is dating my sister(14) but I(M15) feel jealous of her

I’ve known my best friend for about 2 years, we met when a mutual friend introduced us and we hit it off immediately and soon he was hanging out with me everyday. My sister’s only known him for about 4-5 months.

Now I’ve had a bad crush on him pretty much since day one but he’s straight. I told him how I felt awhile back and he said it was ok, and that he felt no different towards me. About a month or so after this him and my sister get together.

Originally when they first got together I felt happy for them but soon all I felt was jealousy towards my sister. I know there’s no point, cause even if they weren’t together he’s still straight. I wanna stop caring so bad, because all it’s doing is making me miserable and I shouldn’t really be jealous in the first place anyways. Every time I see them kissing or doing whatever, I can’t help but just secretly wish it was me and not her.

I don’t know what to do. I don’t know if I should just sit here and bide my time or should I do something?