I'm not a lawyer but read the cases just for my hobbie on this forum. As per the knowledge I have after going through with cases in this forum I can share my view as mentioned below.
But please do not take this as 100% correct answer as I'm not a professional lawyer.
1. My father(Rtd tahsildar in 2010) is facing a problem with my sister in law as she is asking maintenance for her and her 3 children.
Ans. Your sister in law cannot ask maintanance from your father and mother.
2. My brother and sister in law got married (arranged) in 1994 but they are separated from our family in 2002 due to clashes with my mother and mother in law.
Ans. They got seperated was that legally (Means Divorce took place or not?). Incase divorce didn't took place she can ask for maintanance from your brother. If incase divorce took place then in her divorce petition she might have mentioned and may have asked for maintanance or almony. If incase she has not asked for maintanance and took almony then she cannot ask for the maintanance from your brother.
3. My sister in law is very aggresive and like a psycho and she wouldn't give respect to my father and mother.
Ans. Your father and mother has full right to file case against her beneficial if any one of them is a senior citizen.
If incase your brother has not taken divorce from your sister in law but have given any movable or immovable property to her in his lifetime then your father and mother may file a partation suit and may ask for there share in the property which your brother has given to her.
4. She was leading immoral life with another person from then.
Ans. If you can prove this in court then it is beneficial and favourable for your parents and they can file a case of ADULTERY and police has right to arrest that person as well if incase the divorce did not took place.
5. i will divide the inherited property, you and your 3 children with that. But she was shouting on my father to give half part in the property whatever my father earned with his own money and also she wants to 11000rs/ month to maintain her and her children.
Ans. Your father is showing concern and still giving some right to her and her 3 childrens that good. But let me tell you one thing that she do have right to demend share in your father self earned property, But if incase your father doesn't want to give share to her and 3 childrens then she and her 3 childrens has not right left to ask share in property and maintanance.
Your father has full right to take decision of his self earned property he can write a will or can give you or her as a gift the property to you or someone else to whome so ever they want and if she challange in court as well then also she will not be able to get anything from the same as it is your father self earned property.
If incase your brother has bought any movable or immovable and divorce didn't took place then she and her 3 childrens and your parents deserve full right on that property and can divide that property by filing a partation suit.
@Experts: As mentioned above is my views as per the knowledge i have gained from this forum. However, I may be wrong in some points or may be right but still I need your expert views in this query and please do share your views as well and do let me know as well that where I went wrong as your feedback will surely gonna help me a lot in correcting my knowledge as well as enhancing my knowledge.