I kissed my cousin sister
Why would you kiss your cousin?
I got a boner and I wanted to release it
Occasionally people get excited for no good reason, that's perfectly normal.  However, you can't alleviate those feelings by making out with family members.  That's frowned upon.
Can we just talk about my boner and not the kiss.
I'm just saying that it's wrong on every level to kiss your cousin. You should learn to control your sexual urges in the future.