Character A and Character B, two brothers thrown into foster home together, survived because they had each other. Despite the significant (5-7 year) age gap between them. They were separated more than once, but always found their way back to each other. Until when Character A is adopted by a wealthy family, who doesn't want the younger sibling. Character A promises to come back for his brother, to visit, to be there for him but he never does. Until 7 years later when Character A is well enough to stand on his own two feet, can provide for him and his younger brother (who could now currently be early late teens to early 20s), and wants to make amends.
Except, when they meet, their reacquaintance isn't t all what Character A pictured it would be.
Incest, long term/short term (if short term/one shot there would be a heavy dom/sub element) hurt comfort, etc. Looking for the older brother. Willing to change them to not actually be brothers, but just be 'foster brothers'.