Please help me understand Indian family dynamics

Dear Indian women,

I am grateful for this sub and hope that you can help me understand a situation that I find myself in. I am a European woman that is in a relationship with an Indian man. We have been traveling to India recently with the purpose for me to meet his family. His parents are the sweetest people imaginable and they welcomed me very warmly into the family. All could be well. (I am aware that I am incredibly lucky because being welcomed warmly into an Indian family as a western woman is rare.)

However, there is a sister and her husband. And that guy is beyond rude. It is obnoxious. He does not answer when being directly addressed. He sits there and interacts with nobody. He might think that is an aura of superiority but one is tempted to wave a hand in front of his eyes to test if he maybe had a stroke… it's a bit funny in retrospect, but when I met him, he drove me up the wall. He is shit to everyone but he was extra shit to me. He speaks English perfectly fine but when I did ask him something he turned away and spoke to someone else in their mother tongue. He is educated in the best college and I learned that this can make a guy feel like he is the price.

However, everyone in the family just seems to take it. The wedding just recently happened and the brother (my partner) and father of the bride told her that they would support her if she would call off the wedding. Everyone is aware he is a gigantic asshole. There was some demand of a dowry… But she wanted it and now they are married.

In the family there is an understanding that the guy is the son in law and that those guys somehow hold power over the family and that them behaving badly is unfortunately quite common. I really struggle to understand this. It is not like the family would be financially dependent on this guys earnings. My partner is well off and we live in Europe. He sends money to the family. The father is secure. The sister has an education and could further it a bit more to get a better job.

I have now many questions: I have never in my life encountered such an obnoxiously rude person. Like you just wanted to punch his face. Is that common? Why did the father who is a respectable man not call out his disrespectful son in law? What is my position in the family? I am the elder brothers partner (we are not married yet, but being very serious).

One more thing I have encountered is the (more or less) subtle put downs by the little sister. She called me fat on one occasion. (Which I objectively am not.) she ignored me sometimes when I directly addressed her, especially when her husband was around. She tried ordering me around. (Which silly me actually let happen once.) Could there be any cultural context I am missing? Because frankly I was just shocked. I was always being sweet to her, brought her an expensive gift upon our first get together, and just wanted to have a nice relationship with my partners only sibling.

We did not leave the sister and the husband on good terms because I was not quiet about not being disrespected like that. Sister tried to make me budge and is now trying to pull a „poor me, I am married to this difficult guy. You need to take the disrespect for a day because I need to take it for my life.“ Really nobody forced her into that marriage. Like father and brother offered help in calling it off. Is there some other societal pressure that I am missing?

Thank you for listening to my rant/confusion. I would appreciate some perspective. Thanks a lot in advance!