Wiktionary
weak sister
n. 1 (context chiefly US idiomatic derogatory English) A person who is cowardly or indecisive. 2 (context chiefly US idiomatic English) A person or thing which is the least robust or least dependable member of a group.
Usage examples of "weak sister".
He did have the talent, though he was no stronger than a weak sister.
He got blackmailed into working for Schwartz in the first place and he's such a weak sister he couldn't get guts enough to break loose.
This is the era of the weak sister, excepting only Darling and Whisper.
There's boys who say Wayne Junior's a white man, and there's boys who say he's a weak sister.