Wiktionary
a. Not brotherly; not befitting a brother.
Usage examples of "unbrotherly".
Instead of committing these unbrotherly actions, which are so frequent in family life that they might almost be called brotherly, Henry had married a woman who had both money and a sense of repose, and their one child had the brilliant virtue of never saying anything which even its parents could consider worth repeating.
Do you not think the fanatic would certainly find profit in having his most unbrotherly request disregarded?
Garret smiled gleefully and planted a very unbrotherly kiss on her lips.
I think there was a lot of hatred there, plus a lot of rather unbrotherly love.
Soon after Firekeeper had arrived at the Norwood Grant the previous autumn, Edlin had taken a very unbrotherly fancy to her.
And two and a half more years of residing at home, with a sister he was having increasingly unbrotherly thoughts about, had seemed like a living hell.
By this time he had Dytie in his arms and was hugging her with a hungriness that struck Phil as distinctly unbrotherly.