I will tell you by my example. I have a daughter-in-law and she is a really good person, kind and considerate to everyone and always think of my husband and myself as a real mother and father to her.
Sometimes, my son takes a few drinks and come home and just pester her and get angry with her for nothing, and I just feel like hitting him on the head, I'm so mad that he picks on her for the smallest of things. My husband always get mad with him and tell him to get out of the house and leave her alone.
Now if I were to find out that my son was having an affair and the girl is having a baby, I would definitely put him out for betraying my daughter-in-law, simply because she is someone's daughter, she is now under our care and she is above all a good person to get hurt like that.
You have to understand Massi's point of view. She saw her sister (Jai's mother) going through a hard time with Jai's father. She certainly would not want Bani to go through that knowing that Bani is a good person. Remember what Massi heard is that Pia's baby is Jai's and they are having an affair, so definitely she reacted to that. She saw Bani leaving and she had to make a choice of either Bani or Jai stay. Remember also that a daughter-in-law is the responsibility of the in-laws, especially the mother and father and Massi took that seriously.
However, with the same breadth, I think because of her strong anger towards him at the time, she did the shraad, and that I don't feel she should have done, but I understand why she did it. I'm sure she will regret it later.
Maybe for the youngsters on the forum, this might be a little strange but for us older folks, when a daughter-in-law comes into the home, it is our duty to protect her and treat her just as we would our own daughter.