Sister 19 [F] of a friend continuously sending sexy pic

Since some time ago when I give a ride to the sister of an old friend of mine, she started to send me sexy pic of herself, out of nowhere. It’s not something that usually could bother me but it’s really difficult to manage the situation. She ask me to send some dirty pics of myself but i didn’t do and ask her to stop, but without results. I mean I appreciate the photos, but it’s really a difficult to manage situati