This reply is not very helpful, but I hope you find the manga! It sounds really interesting and I also want to read it (≧∀≦)
Maybe Toshishita no Ryuugi?
Just a guess but maybe Toshishita no ryuugi?
All I could think of was http://www.mangago.zone/read-manga/toshishita_no_ryuugi/
lol (≧∀≦) two other people think so too!
TY so much guys ♡ ～('▽^人) ,this is the one. I started it but then lost it so I couldn't finish reading. o(TヘTo)
Non-blood related brothers living together (younger bro moves in with big bro at the start of the manga). Older bro is a model. Younger bro in love with older bro. In 1st chapter younger bro sees older bro half naked and gets really flustered.
I've been searching everywhere to find this manga and need help! >_<