I promised my sister I would come out?

I'm 13 and I've resonantly realised I'm gay. The other day my boy friend came round and because we thought no one was home we went up to my room and started kissing, then my sister burst in. The next day I went to talk to her about and she agreed not to tell any one especially my dad or my older brother (my mum died a wile ago) Earlier she asked me to tell them tonight because it's hard to keep a secrete like this and she's not sure she can hold it in much longer she said she's scarred of saying some thing in and she doesn't want to keep a secret any more. I'm fed up of lie as well so I said yes. But now i'm freaking out. Any advise would be appreciated