Need help about by sister's troubled marriage

I 30(M) am in a difficult situation right now because my sister's marriage is on verge of breaking.

She (34F) got married in 2017. She had her first baby in 2021. Now in 2024 she had her second baby. Her inlaws and husband are mentally torturing her because she could not give them a son. We talked with them but all in vein. It felt like our words fall on deaf ears.

The second baby is just one month old. Currently she is staying with my parents in mumbai so my sister and her daughters are safe.

As a brother i want to ensure safety of my sister and my nieces. I stay in delhi for my job and i will be getting married soon with my GF.

My sister is educated and used to work till 2022 in mumbai. She is academically good and hardworking person so she can earn a living for herself if i support her. I want to be there for her always. However, i do not know legal aspects of divorce. We need her daughters with us. We cant let her family take any one of them away from my sister.

Financially, i can take care of things till she is settled. I request you people to guide me in this sutuation. I just want safety for my sister and my nieces. What course of action should i take hereafter ?