Mizz Khan,
In order for your problem/question to receive better attention, my suggestion is that you should create a separate thread. This is As-Salik's thread and it deals with her particular problem and she can benefit from all the advice that people give her regarding her problem. Your post.....might cause people to become distracted from As-Salik's problem....and they'll end up giving YOU advice on **HER (As-Salik's) **thread, which isn't fair to her. My intention is not to offend you. I'm just trying to help you get better attention for your particular problem....and a separate thread will keep the information more organized for you.
Now about your problem......I've read your post and here are the possibilities that I've come up with:
1) I do sense that there is some jealousy that your sister has toward you because she didn't congratulate you when you got a job. And jealousy between sisters/siblings is very normal.
2) Your sister's mother-in-law (MIL) and your fiance's mom are SISTERS and the BOTH DON'T GET ALONG. Maybe your sister's MIL is constantly complaining to her about your fiance's family. And maybe your sister is SCARED that if you marry this guy......then this will create problems between you and her (your sister) because both families don't get along. In other words, she doesn't want the family drama to cause problems between you and her. Think about it. Tomorrow what if your sister's MIL tells her that she is not allowed to see you or talk to you........because of problems between the two families. Your sister could be afraid that the family drama will affect the relationship you both have. Maybe this is her way of trying to protect you.
3) Maybe your sister's mother-in-law is giving her a HARD TIME because you are marrying the son of her enemy. Sometimes mothers-in-law take out their anger on their daughters-in-law. Plus, you don't live with your sister, so you don't know how her life is like. Maybe her in-laws are creating problems for her because you got engaged to a family they don't like. And to them this might symbolize some pathetic kind of betrayal.
SUGGESTION:
Sit down with your sister and talk to her calmly. Tell her, **"Look, Baji, you are my sister and I love you. I'm not your enemy and I have no desire to be one. If we can't even communicate now as sisters, then how will things go for us in the future. You're older than me and I know that as my sister, you would want what you believe is best for me. So, please........tell me honestly, what is going on here. Are your in-laws giving you a hard time because I'm marrying your husband's cousin? Are you afraid that if I marry into a family that your in-laws don't like, that this will cause problems between you and me? Are you afraid that my rishta is impacting your marriage? My fiance seems like a nice person to me. He hasn't done anything to hurt me. And I want to believe that your opinions are not out of spite toward me....but rather out of love. So, please open up to me and tell me what's bothering you."
**