What should I do about my sister?

My sister is REALLY freaking me out. I'm 14, she's 16. Her BF lives in Tennessee and we dont, so she video chats with him on skype. My step mom and I have caught her a few times getting naked if front of the camera! I just heard her scream a sorta hugh pitched eep and i think it was an ! OMFG. What do I do? help me, she's never done anything like this! What should i do?!