Why would he fuck the mc if he thought of him as his older brother
From what I understood Sonny was just worried about him because he thought he would end up dying due to Leo's brother just like his own older brother. I don't think he ACTUALLY though of him as his own older brother, unlike Leo who had a crush on the older brother and only became interested in mc because of it (ughhh I still don't like Leo). So yeah I think Sonny could have sex with mc exactly because he could clearly see how different he is from his brother since the beginning and didn't overlap them (unlike Leo. SIDE EYE). I don't know why I'm even over-analyzing a smutty 3p yaoi LMAO
I didn't finish this yet but I want to rant about how I hate hate HATE this replacement trope going on!!!! Leo is clearly only interested in mc because of Sonny's older brother, his first love. Sonny is most likely also seeing his own brother in mc but not in a weird way (?), he's just worried he's gonna have the same fate from what I'm seeing. If I didn't already know that the ending is poly (yup it is, I saw the ending) I would 100% root for Sonny to be the endgame because tbh apart from Leo being very hot I don't like him one bit so far