The biggest question in here is... In the future when the boy grows up who will be the top...

if he even grows up during this story, my bets on the ponytail being the top

I vote for big bro cousin as seme I feel that the boy is the type who will not grow that much plus personality wise the boy is more of an uke as the cutesy naive type while big bro cousin look the responsible big brother type seme(▰˘◡˘▰)

I was hoping for the niichan to be the seme as he seems kinda reliable and somewhat mature, but i also couldnt stop thinking what if the ponytail boy becomes a semeლ(´ڡ`ლ)