I can't get along with my fiances sister?

SORRY, THIS IS GOING TO BE QUITE LONG. SKIM THOUGH IT IF YOU'D LIKE, THANK YOU KINDLY!

I tried so hard already, but I failed miserbly and so, I think I give up.

I'm out of options and she's honestly not normal at all. Nothing she does ever makes any sense at all and half her family agrees with me, but they still love her and get along with her! The girl hates me, is jealous of me and its just annoying.

I don't know how to get over not talking to her because it upsets me to have an enemy on my fiances side. I mean, I don't even think I'd invite her to the wedding because it would hurt me to see her there and she'd probably throw nasty remarks my way.

I was at her wedding (she got married at 19 to a man age 25 within 5 months of dating) and she told me that she's happy for my engagement, but that she's not certain I'm right for her brother! Also, I once CAUGHT her looking at my photos on my fiances camera and deleting them! I asked her what she was doing and she told me she accidentally deleted them! These were really good photos of me too. They were taken in a park thats 2 hours away from me with a beautiful bridge behind me. She has yet to apologize for that.

Not to mention she deleted me from facebook when a guy friend of mine who is good looking wrote on my page "you're so hot it's scary!" and i wrote back "i know, it's so hard being hot i scare myself all the time, lol"

When I confronted her about it and asked why she deleted me, she told me it was THE VANITY of me. I am nowhere near vain. I even asked 12 of my friends if I was vain and they all said no.

I also caught her talking about me on the phone with a friend while I was over my fiances. She said "___(my name)thinks shes so hot, but shes not. she also wears way too much makeup and photoshops her photos. what a loser,right? i don't want her in my life i dont need someone like her. why couldnt lilly marry my brother? shes so much better" - lilly is a girl who isnt attractive at all and whom has never had a boyfriend. I love the girl, but see my thinking/hers?

First off, I only wear mascara and lipgloss while she wears eyeliner,concealer,blush, you name it.
Secondly, I don't even own photoshop nor do I know how to use it.

This hurts so terribly. What should I do? I tried to talk to her and ask her how she feels about me and she just said "let it go". I am ALWAYS nice to her. I want her to like me, you know? I am trying hard to like her!

Please help?