Okay, It's a long story, first of all IM REAAALLY sorry for all the grammar mistakes and please I know I've done wrong things, and I messed up, don't say bad things about me because I already understand the wrong stuff I did.
So, it all started when we were chatting (me and my sister) with my cousin's best friend who is on America and we are in Greece. It all was fun and stuff, I started having a crush at him but he was looking at my sister more than me.. but after some time, when we got home with sis, she told me that she became really good friends with.. lets call him david.. and she thinks he likes her but she really doesn't.. and I trully believe her, she would never lie to me.
So, they talked for hours and houurs, I tried talking to him too, but he wasn't interested so much to me. He was very kind but.. I could just realise that he just wasn't interested.. after some time, my sister said that she doesn't want him to ruin our relatioship, because she found out that I liked him and she blocked him from Facebook, deleted his number,skype name and EVERYTHING.
Two weeks passed and I just couldn't help myself.. I made a new Facebook account and I put my sister's name, I added david and told him that I was my sister but something went wrong with my old account so I made a new one.. He believed it... and now we are chatting for hours and hours.. he believes I am my sister and he keeps talking to me and I'm really happy, you can't even imagine, I love him so much but I know that he doesn't love me for who I am, I am supposed to be my sister.. What I did was wrong but I cannot stop..
How will I tell him that I'm not who he thinks I am? Will he freak out? Will he keep being with me? WIll he get too mad?. I really am sorry about everything I did..

Doesn't matter what HE will do, you MUST come clean and that starts with being honest now with this fellow AND your sister. Maybe tell your sister first!!

If you are truly sorry you end this selfish deceit, and tell them ASAP!!!!!! No Excuses

Yeah really. When you lie and deceive, you create a mess that won't get better until you become honest. And this is how people get hurt too.

He's going to be ticked off... and so will your sister... you can't undo what was done... so come clean with both of them.. they are going to figure it out eventually anyway and its only going to end up really ugly... the longer you decieve them the worse its going to be.

Only they can decide about if or when they will forgive you. But expect it to take time. And find someone else thats not involved with anyone...not just a friend or family member.

You need to come clean. He will be mad and so will your sister. He may not ever want to speak to you again but that is a choice you made. Deal with the circumstances, learn from it and move on.