Jeez banana, I can't believe you're bringing this up again as far as them being a gay couple.
Uncle Charley was indeed Steve uncle by marriage. Charley was the brother of Steve's father in law, Bub.
Uncle Charley was the boys great uncle.
Steve was widowed and needed help with the boys, so in the beginning of the show Steve's father in law Bub was helping out. The actor Wm Frawley who played Bub became sick, and was replaced by the character Uncle Charley.
Steve, a widower, eventually remarried Barbara, a widow. Barbara had a small child Dodie.
It is wasn't for the fact Frawley became ill, the character Charley never would have existed.