What do i do help me pleas!?

ok i just turnd 18 a few days ago and a girl that iv liked for a few years just broke up with her bf and we started talkin but shes only 15. well to today she wanted me to come over to her house while her mom was at work so i did but we have never kissed or done anything relating to at all. all we did was laid on the couch and watched a movie together. well my step mom found out and told her mom we were togather at her house and my step bro called me and told me we were cought so i left and now her dad is thretining to beet her and i dont want her to get n ne trouble but we want to c eachother what should we do? pleas help