If you're a new in-law, you better be on your best behavior.

Meeting your significant other's family can be a big deal. You probably want to put your best foot forward and make sure that you make a good impression on them. First impressions can be a big deal, and that can follow you throughout your relationship with somebody. Odds are, you want a good relationship with your in-laws. Even if you don't see them that often, it's probably important to you that they hold a high opinion of you and you hold a high opinion of them. Even if you don't always get along all the time, and you have different outlooks on life, you still want to be in each other's lives. If they're important to your partner, then they should be important to you. And it's on you, as it is in law, to meet them halfway. You don't have to believe everything they say, or even be on the same page about basic life stuff, but you do have to respect each other. Respect is the bare minimum for anybody that you encounter on this earth. The sister-in-law in the story didn't even have that, though. She, for some reason, had a very tense relationship with her future mother-in-law, and she was not doing anything to make that better.