Younger coworker won't take no for an answer

I 28M work at a small family owned business part time 2nd job. The female coworker (F18) was the one who interviewed, hired and trained me. Before I knew how old she was we'd often tease her often about work related stuff and I thought nothing of it until she started getting touchy. After i found out she's only 18 and distanced myself. A few months passed since I tried to let her down lightly. I told her something along the lines of she is sweet and attractive and I admire her work ethic/personally as a whole but she's much too young. The other day she sent me a video her in the shower. There's no way it was a mistake as it was addressed to me. I just deleted our text history completely. I can't block her because she's one of the boss's daughter and pretty much the manager. Today she asked if I was mad at her and asked why I haven't responded to or been talking to her.

I'm not going to lie when I first started I thought she was in her mid-20s because she said she was going to college to become a doctor, is the manager and was working 10-12 hour shifts just like myself. Even her older sister noticed and mentioned to me that she has a thing for me.

I don't want to lose my job because I quite like it and make easy money. I don't really want to talk to my boss about it. I'd consider myself an introvert and normally don't speak unless spoken to. I don't really have any idea how to handle this. Any suggestions?