What should I tell her?

My sister is 17 and I am 28. She wants to have with her boyfriend. I told her that she is too young. But she will not believe me. I am a wife and mother of 10 mth old twins. She said that I am lying to her. My husband and I first had when we got married. We got married in the beginning of August. We had Thanksgiving weekend. We gave birth on August 21,2005. She said that I am being to over protective. What should I tell her. Thank You. Please no rude answers.