Can someone tell me if this is gonna end up being romantic or sexual??

I think romantic? Since it's shonen ai, not yaoi

It depends on where the author wants to go with this. I think at least it's going to be romantic. I myself am waiting for Tsubaki to grow up and get to the wild rebellious phase of teenagers. I want to see him be rebellious (and maybe chase after Yuu-nii.. I really want a kabedon scene XD) Yuu-nii is already a third year so they might separate in the future if Yuu-nii goes to a uni far away.

u.u this is gonna be slow burn.

Neither