"X你妹" (lit. "Moe your sister", X can be replaced with many chracters) is a popular phrase in Chinese youngsters. It can be used to describe your strong disagreement or anger with somebody's words (especially on you), but in a joking way.
"X你妹" is a variant phrase derived from "X你妈" (A nationalwide f-word in Chinese), which is a very offensive abusive word means "Fuck your mother" or disagree people's words in a rude way depending on the character X. People who want to express their thoughts in a strong way but without hurting the others or starting a fight, they began to use this.
e.g. "看你妹"(lit. "See your sister")=見るな＝What the hell are you looking at!/ Stop eyeballing me!
"好你妹"(lit. "Good your sister")=良くないよ=Awful, terrible.
Or the most simple usage, "你妹", ignore the Character X, to show your disagreement or anger to the one who talks to you, if you think his/her words are nonsense.
