What age did you start sending nudes?

My sister just turned 14 and I really think she is sending nudes to her boyfriend. I also know they have gotten more.. comfortable with each other... there have been a few times where I was uncomfortable because I was supposed to be pretty much "keeping an eye out" while my parents were gone. They were in her room and the light was off and it was silent for like 30 minutes. So I'm trying not be the crazy older sister. But they act guilty when someone walks by there room. I don't want to make her feel like a whore and I don't want her to shut down if I talk to her.... but I'm also trying not to freak out... I started sending nudes when I was about 16. And it was not smart! Advice?