lake I dont know if I'm ready...?

well im 13 and my girlfrend says she wants me to be the one hoo takes her virginity. We've been together for 8 months now and last time I saw her we were really close but my sister (who is 8 years old) walked and told my mom. but next time we see eechother it just might happen. I want too but you know i just dont know if its the right thing to do. I'm probably putting to much thought into this but any input would help.there