Problem with Fiance Sis in Law?

Please help me I have certain problems with my fiance's sis-in-law. She is a wonderful person no doubt, but at certain times she moody & behaves weird. When I was introduced to the family, initially she was jealous of me. I can say this because whenever I discussed about clothes or anything she made it sound as though she's smarter. I'm a Fashion Designer and she made me sound bad at times. For example once my fiance & I had a conversation at their place about why people not buying clothes. She said that why would anyone buy from you Designers when there's cheaper stuff in Bangkok & China. She laughed & went in, did not even let me justify myself. When we went to Goa she showed some of her clothes & I showed some of the Bridal lingerie that I had. She giggled in front of her sister & said that she has worn much more than these. She did not acknowledge me in the entire vacation at Goa & kept giving her sister company. I felt lost & alone. One day in the kitchen she tells me that it was my fiance that her family liked & wanted her to get married to. She then said that her mother said it's OK what you have chosen you have chosen. I was annoyed by what she said. The entire family keeps praising her for the food she cooks & many things. But I don't react because I'm not jealous. But whenever I go to their place she does not let me help her in the kitchen. All she has to say is "Chill". Even when I don't ask her if she needs help & sit quietly after greeting everyone. I say "HI" to her and she replies Chill. I asked my fiance bout this & he finds me overreacting??? He also said that I'm jealous of her and also said that I'm hyper. It's a joint family I'm going into & my folks were against it. But I said no because I liked the family. But her behavior annoys me. If she has been with the family for 2 years & settled. She should also understand the fact that even I'm going to be part of the family. I'm not stealing her place. But going to be part of the family. Even I want the other family members to accept me. At times she shows her new stuff that she buys & I show her that too. I share things with her & she does the same. She helps me straighten my hair & I help her with the make-up. But sometimes it's just too much. It also annoys me. Please help. How can I be calm & enjoy with the family. Also how should I behave with her? She's a Gemini Yes Gemini. Very Moody, very two-kind. I want to have a good relationship with her & all the family members in the future.