Tell or Keep It Shut?

Okay,so my mom doesn't allow boyfriends,but my sister is 14 in the 7th grade and has one,but my parents don't know. She told me to keep it a secret. My two older sisters are even involved in it! They pick him up and take them to the movies,take them out for ice cream,ect. But,we just in an argument. She called me ungrateful and a tattle teller. I told her I would tell my mom,and she said she didn't care. I was jealous that I didn't have a boyfriend (which I don't,cuz I just got hurt by one and I'm not up for mingling right now). But,I was never jealous and that got me reallly mad! So,when I got to my moms door knob,I had second thoughts,and wanted someone else's opinion before I make this BIG decision. Please,please,please help me. I NEED HELP!