People who think Oswald saying he and Ed are brothers will stop me from shipping them together are funny.
The writers wrote them as a romance and wanted to make them canon. Except the producers stopped them. So there's nothing brotherly about their relationship.
Especially since that "we're brothers" line was forcibly added by the producers on the day of filming, which the writers didn't appreciate.
In this context, I take "we're brothers" as brothers-in-arms. Like they fought together, simple as that. Not in a familial sense. With everything in the show, within its narrative, that line can't be interpreted any other way.
Especially since I watch tons of CDramas where people who fall in love literally call each other brother and sister as a sign of respect and also because they learned to fight together under the same master.
So I don't get that supposedly familial connotation with Oswald and Ed. Because she doesn't exist.