Hello, Hany, and welcome to the Grammar Exchange.
@Hany Younes posted:
" Her mother is more helpful, isn't it? or isn't she?
@Jack Milgram posted:
Question tags form from the subject of the sentence. The subject of "Her mother is more helpful" is "her mother." "Her mother" is an animate feminine noun. That means it turns into "she," not "it."
Jack is right that the tag question should be "isn't she?" I'd just like add, lest there be any confusion, that "her" has nothing to do with the need for "she." The tag would be "isn't she?" even if the subject phrase were "His mother."