Even if your bf was as open to polyamory as you are, he might have a "messy list" of people he would prefer you not date. His brother, his father, his best friend, his boss. Cousins could certainly be on that list.
I've been poly at heart my whole life. However, I lived mono for 3 decades with my ex husband. I wasn't being true to myself.
I used to get crushes all the time. I pretty much was always crushing on one person or another. On celebrities, friends of my husband, neighbors, other women in my social circle, etc.
It was a way of being poly without being poly. I'd get a crush, and masturbate while fantasising about whoever my current crush was. Often I preferred doing that to actually having sex with my husband.
All the crushes passed in time. It took longer with some than with others.
Of course, billions of ostensibly mono people deal with this. Either crushes, or porn and fantasy augments their supposedly, outwardly, monogamous sex life.
If you really want to create polyamory with your bf, you should try. If he can't deal, you could break up, and start dating people who are also poly, or monogamous but poly friendly. Then you won't have to suffer with these fun but painful unrequited crushes on inappropriate people, such as your bf's cousin. You can date others cleanly, and actually get to have sex with them.
I'm guessing your crush developed just now, at 2 years into your relationship with bf, because the final effects of NRE (new relationship energy) have just passed.
This is what I manifested in my life: My husband and I broke up (for various reasons, one of which was my poly nature). I get fewer crushes on unattainable people now. I just date real people that I like! And I don't avoid sex with my nesting partner (my gf) to go off and masturbate to the thought of someone else. I have plenty of sex with her. I also can have sex with whatever other person I desire. And yeah, sometimes if my gf (or current bf) isn't around, I do pleasure myself with a fantasy of someone else, with no guilt. Generally when I have a bf who comes over twice a week to bang me, I don't have much need to masturbate. Twice weekly sex with a bf, and sex 2 of 3 times a week with my gf is pretty satisfying.
If I were to get a crush on my gf's brother, or her best friend, say, I would keep my distance and not feed into the fantasy more than I could help, and wait for the crush to pass. It's OK to feel feelings, we all have them. It's how we act on them that counts.
I'd recommend you don't tell bf you are crushing on his cousin, especially right now, when bf is feeling sad about the thought of poly. He is in a lifelong r'ship with that cousin, you don't want to taint it.