A resume for those who are having difficult to understand their relationship:
1- They are not brothers. Not half brothers. Not even close to each other
2- They first met on a orphaned while kids. Were addopted by different families and spent years without see each other
3- They met again after many years when the couple who addopted the bottom died on a accident. Bottom around 16y and top around 24y old. Is this ocasion top asks bottom if he would like to be his brothers since he lost his family. Bottom agree
4-After that they live around 5 years together. But they never get close of each other because bottom was always trying to suicide. He even hurted top with a knife at one of his suicide attempts. Top kinda says "want do die? ok... not trying to get any close of you". So they were never anything close to brothers, not even on a afetive way. 
5-At the current scenes bottom has around 21y and top around 32y. And they both clearly have some mental issues and need therapy