Thomas Raith lived with Harry during his training at cosmetology school, but I do not remember any reference saying that they actually had a sexual relationship. They both knew they were brothers at that point. I do remember Harry visiting Thomas's shop later, the staff assuming they were lovers, and it being something of a joke between Harry and Thomas.
It's both joke and obliviousness. Two men being roommates? Especially when one is (supposedly) a gay hairstylist/a White Court 'sex god/incubus' vampire? How can there not be anything going on?
Put it simply, the idea of a Wizard and a Vampire being brothers is something the White Council isn't quite ready to accept yet. I bet it's the first time (at least since Merlin) that's it's ever happened.