You’re being overly dramatic: thought he was married or something! You say there’s a significant age difference but then only a couple of years.
The only issues are that you live 5 hours apart and you don’t know if he (1) fancies you and (2) would consider a relationship with you. Chances are he wouldn’t, given the long distance.
Your friend may not even think of her brother when it comes to matchmaking friends or might not wish to in any way matchmake him with her friends, eg due to concerns about potential awkwardness.
In your shoes I would just unfriend him on social media and move on.