What should I do about this guy I like? He talking to me but he trying to talk to my younger sister too.?

Okay so I am 17 and my younger sister is 14. We met this guy about two weeks ago he is 17. He started talking to me but he has been secretly talking to my younger sister behind my back, I told him how I feel and he understands. What should I do? Move on or just continue talking to him.