I love my boyfriend but there's just one problem... My sister. My sister is always texting him and messaging him and always flirting with him in person and tells him things about me that aren't true at all and he talks to her on Skype more then he talks to me and my sister never lets me talk to him. I don't know what to do about this situation but I'm getting jealous and mad at the same time please help me!

OK, I'll help - DUMP him like a hot potato. Don't get angry, don't complain, just tell him it's over. How's that?
If he begs and apologizes, make him sweat it out for at least 3 days.
If he shrugs it off as nothing, he didn't care, and you are better off.
If he gets mad at you, he wasn't worth the effort.

Your boyfriend should have set the boundaries if your sister has been the aggressor, and your sister should have set the boundaries if your boyfriend is the aggressor. Since neither have set boundaries, you can't trust either of them because they are both getting something out of this.

Break up with the boyfriend and think a good long time before you introduce your next boyfriend to your sister. I think it would be appropriate to discuss the skyping and all that's been initiated by your sister to your boyfriend with your parents. This girl needs some guidance on how to be a sister and how to conduct herself with boys, and better to come from her parents than from you.