Why does my boyfriend's sister hate me for no reason?!

My boyfriend and I have been together for a year and a half. We are very close and have met each others families and gone to a few family gatherings. We also sleep over at each others houses as well. He is always all over me - hugs,cuddles, caresses, kisses, etc. even in public and around his friends! people even ask us if were a married couple!

His family likes me very much - they say I'm a thousand times better than his ex - my personality and looks (his ex was a bitch, plus me I'm their own kind). Everyone in his family has welcomed me with open arms and they all recognize I'm a good girl/ wifey material. I have always been nice to all of them - except I'm sometimes very shy (they all know that)

At Christmas time my boyfriend bought me a real diamond pendant which his younger sister saw...

My boyfriend gave me the OK to come over at x-mas time , but for some reason his sister was in a very bitchy mood all day long and she was very rude to me (her excuse was that I didn't say "hi" to her, but I had a feeling she was moody that day so I kept quiet). His sister was even banging the kitchen cupboards and his mother was telling her not to do that. She was very rude to me that day for no apparent reason... I never gave her a reason to not like me and I was always very nice to her.

p.s - my boyfriend and his brother received more money than her at Xmas...
Jealously/attitude problem
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She feels I'm taking her brother away from her
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Why does my boyfriend's sister hate me for no reason?!
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