Legal value of notarized stamp in court

My sister married in June 2014 with person (within caste) from Surat, Gujrat. But soon after marriage, their in-laws (especially mother in law) started mental torture of my sister, by arguing about her parent's status, and always criticize her regarding quality of work/ quality of cooking. They always insures, that she should not get time to rest, after hectic works, So they asks my sister to clean home at least thrice a day. Even she didn't gets enough time to finish her meals, she had to leave it incomplete most oftenly. Though they did not asked clearly about dowry, but they always argued that "What your father has given to us in marriage???". We tried to sorted out this matter, by discussing with my wife's husband & her in-laws. But it didn't helped her. She spent around 6 months in the same situation. Then she become pregnant & came to our home for first delivery. After 9 months of marriage, my sister delivered baby boy. As it was cesarean delivery, so she had to rest for 2 & half month after delivery. And then she returned back to surat. Now, though she has little baby with her, their in-laws's attitude didn't changed about her. Things became worse than earlier. It became very hard for her to take care of her baby boy. So, finally we called her back to our home along with her baby boy. She stayed almost 3 years with us, and in mean time she kept contact with her husband. They used to talk everyday over mobile phones,he always puts pressure on her to come back,but he never assured that all conditions will be changed, that time my sister put one condition that, she wants to stay in separate home, away from her in-laws, so that she can pay attention on her child, then only she will come back. Her husband didn't accepted it for atleast 3 years, but few months back, he saw a measure loss in business (of textile) due to note ban & GST, so he had to packed up all business, and started to set up a new business. But in meantime, he had to stay dependent on his young brother, for daily needs & spences. Financially, he become so weak, he was not able to give a single rupee for home spences. Even their family member, didn't helped him financially and in reverse they used to blame him for all his business loss. Recently, he started online sell of clothes, and started to gaining enough amount. And now he don't want to stay dependant on his brother any more. So, on his own, he ready to stay separate. So he rented a home, and insisting my sister to come back to Surat. But as time of 3 years has passed , we need ensure safe side of my sister & and her son, she should not face the same old conditions, Or someone from her in-laws, should not give her mental/physical torture. So we are thinking to put all these assurity on notarized stamp paper and get it signed from him. Does it legal in court, if we want pull him into court in case of any problem?? And should it signed from all in-laws members?? If it is not legal, please suggest legal process.