My sister and myself are getting each others text messages. Help! (Hard Question)

This whole problem started when I first got my Iphone 5 around christmas time of last year. And i did the normal activation of my new Iphone and syncing my old iphone 4 with this new phone. I synced everything and it was all good, but i started to get these random text from people in my message log. I soon found out that i was recieving every text that was sent to my sister. And if i replied to someone who was texting my sister it would show up like she texted them, not me on a seperate number. Also when i text my sister she would get the text but immidiatly after i sent her it, i would also get a text from myself saying what i sent her. I will literally get a text message from my own number to myself, (Weird ****). So its like my sisters text messaging is fused with mine. Its only messages though, no images or anything else. In the beginging i didnt care about this becasue she wasnt recieving my text i was only getting hers. But now just today she tells me she is getting my text too. And thats a problem! This problem lasted all the way up to about 2 months ago and then i stopped getting her messages. But i gave one of my friends my old iphone 4 and once they activated it for there own use it started to happen again. They use a seperate plan then i do and there number on the phone is different. Before i gave them the phone i wiped everything off of it. i took off the contacts, messages, everything. They use a different apple account and icloud account now on the phone. So idk why it started up again i need help. Here is some background. My whole family shares the same apple account with everything. We have seperate phones, numbers and data for the most part. But with dealing with Icloud and itunes anything like that, we use the same account. My sister uses a Iphone 4s btw. With my old Iphone 4 i had no probelms with this, everything was normal. But once i switched to the new iphone 5 this problem has occured. Idk what this is about, i havent heard anyone else with this problem, but i need help! btw my other family members on their phones do not have this problem. So i think it has to do something with the icloud but im not sure! This is a serious problem and i need help! Goodluck!