In laws torture

Hi, It's been 5 yrs since my sister got married but immediately after marriage she was being tortured mentally by her in-laws! My family intervened into the matter and tried to resolve the dispute but all efforts were useless. Now it's been 4 yrs that my sister is living with us. She has a 4 yrs old son as well residing with us only since his birth. Neither my sister's husband nor her in-laws visited us for initial 3 years of dispute but from last one year they are trying hard to get back there son and trying hard by several means to convince my sister to ask her to come back. But now my sister don't trust them at all and refused to go back by saying that it might be a conspiracy from them to get hold on to the baby. Mark my words, it's definitely a plot by them as they are really insensitive people. My sister is working as a teacher in a private school and happily and efficiently raising her child with my parent's support. Now we want to end this chapter by legal means and also want child's custody as he is the prince of our family. Please guide me the best legal ways to fight this situation. What are the legal chances of getting them punished as due to the mental torture my sister is still somewhat in a trauma and on regular medications to overcome the depression of a failed marraige life? What are the rights of my nephew onto his father's property and all? Pleas guide! TIA