Rene,
Here is what I think. Keep in mind that your problematic sister-in-law is the YOUNGEST in her family. She's the baby in her house and most likely the "laadli" **of all the siblings. And many times...the youngest person in a family receives the MOST** attention and the **MOST **pampering. And unfortunately some parents let the youngest child get away with misbehavior that the older children are persecuted for. Some parents will brush it off by saying...."Koi baat nahin. Choti hai. Naa samajh hai, bla bla bla." I'm not saying that this attitude is right.
What I'm trying to tell you is that the youngest sister-in-law most likely feels insecure **and **JEALOUS of you. Think about it. She gets all the attention in her family because she is the youngest............and now YOU are getting her family's attention because you will soon become a new addition to their family. YOU are the "new interest" in the family because you are the "hone-wali bhabi" the "dulhan-to-be"...........and this girl is feeling jealous about the attention that you're getting.
In my opinion complaining to your fiance is not going to accomplish much. As a result, she might even become more resentful toward you in the future and cause problems for you. Just try your best to be patient with this girl. And continue being nice to her. 20 is a very young age and you are 9 years older than her. She does not have the maturity that you have. Guys and girls in their early 20's can be VERY immature in their attitudes and careless about their conduct.
You seem like a caring person who wants to establish a good relationship with her in-laws. But keep in mind that you are going to marry your fiance.......NOT his sister. And he is happy with his decision to marry you. His mom and other sisters also like you. Please don't get so stressed over one immature sister. The other sisters' behavior is still positive with you. The other sisters are not being brain-washed by the youngest sister.
So, just continue being polite to her. You are older than her. And instead of losing control and getting mad at her.......SHOW that girl what respect looks like. DEMONSTRATE respectful behavior for her. After you get married.....if she still continues behaving badly....then talk to her directly. Tell her, "I'm not your enemy. I'm an addition to this family and I'm like your sister. I want you to feel comfortable around me. I'm not a threat to anyone and would like to fit in. So why can't we be like sisters and friends?"
Or after marriage.....you can even tell your Mother-in-law "Aunti/Ammi......I think (name of girl) is a really sweet girl. And I see her as my sister and would like for her to feel comfortable around me like your other daugthers. She seems a bit shy around me." If you say it like this.........then you won't look like you're complaining and will seem genuinely concerned. Maybe her mom will encourage the girl to open up with you. But after you get married.......talk to the sister-in-law first....and if that doesn't work.....you can gently mention it to MIL.