Is there a problem for a 19/20 year old frum girl giving private lessons to her 12 year old cousin who’s not religious(teaching him Hebrew and about Judaism)?
I am assuming that you are talking about giving him lessons in school work etc. and not anything physical. Therefore it would be alright, however you must make sure that you won’t be in a yichud situation. Secondly, make sure that the ambiance of the lessons will be business-like and that it should develop into a situation that you get to close or friendly.