반가워요! Q: "걔의 엄마는 때문인가?" "Is it because of their mom?" A: (Peer speech) 그게 걔 엄마 때문이야? (Polite speech) 그게 그 사람(의) 어머니 때문이에요? (Honorifics) 그게 그 분(의) 어머님 때문입니까?

I was trying to say, "Is it because of their mom?"

Good! but I couldn't understand exactly. What do you mean by that?