7 yr old asked me about sex?

Okay so im 12 and i have a 7 year old sister named Alexis and a few days ago she walked in my room and said Kam can i ask you something and i said anything baby(i call my sis baby) and she said My boyfriend asked me if i would have sex with him . what is sex? i jumped off my bed about too cry! i just said Alexis give me a minute to calm down. and she sat there while im walkin aound my room asking my selfover and over again what the ****!? I started too cry . Then shesaid are you okay? and i screamed at her and i said GIVE ME A MINUTE! our mom was on a business trip for 5 days and our older sister was out at a party when she was supposed to be watching us. and i wuldnt dare tell my mom or my older sister! What do i do!? im soo worried! i dnt wnt her to follow my footsteps bc i had sex with my sisters boyfriend when i was 9! plz help me:/