I have a small crush on my boyfriend's brother

Let me start off by saying its not like Im 13 and hes 27. Theres only a two year age gap and I believe thats legal everywhere.

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When I first met him (High school, he's held back a year so hes a year ahead of me i.e. Im a currently junior/11th grade and hes a senior/12th grade) I didnt have any thoughts on him other than "This is my boyfriend's brother and possible in-law" But after two years, I had a different outlook. Everytime I saw him in the hallways and said hi, my heart would skip a beat. When he would sit and chat after school with his brother, I would get lost in his voice and enjoy his scent. I remember getting him a treat for his birthday since its all I could do at the moment, he said how he sees me as a sister and that sentence stuck with me. I also remember at some point when I was in a voice channel on Discord, he sat with my boyfriend and talked with us. He mentioned how he was going to go hang himself (As a joke, please dont hate comment) to which I replied that I would join him that led the conversation to him hanging his brother, the both of us having sex, one of our friends masterbating in the corner and the other had killed himself beforehand. I dont know why these thoughts stuck with me and why I keep thinking about him more than I can think of his brother instead. People would mention how my boyfriend is less physically attractive than him but I cant agree or disagree on that.

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Ive been with my boyfriend for a year and I secretly masterbate to his brother in private. I dont have much contact with him other than him occasionally asking where his brother is after school with me. Ive been told to delete his number but I cant for some reason.