Question

which phrase does not agree with the verb in the sentences wins every dance contest. my sisters, her sister in law, the step up sisters captain, my sister

Answer 1

To me it would be her sister in law which is a phrase I don't agree with

Answer 2

Just put each phrase with the word and listen to which one sounds wrong. I would have to say "My sisters" does not agree because the verb win has an s at the end. One would not say "My sisters wins..."
Every other choice agrees with the word wins...
"Her sister in law wins every dance contest."
"The step up sisters captain wins every..."
"My sister wins every..."