Sister left home with mother and also not letting to speak with her

Hi, My sister who was living in Kolkata with my mother and her son has suddenly left for Bangalore without telling anyone. During the last phone call early January she told me that she cane to bangalore and will not tell anyone where exactly they are. She was speaking ill of my mother and told me to come and take her away. Since then i am trying to reach them but she has blocked my number or changed her number and is not picking up my calls. Also to note since months she has not been letting me talk to my mother. I am abroad and need guidance on what should be my next course of action to be able to reach them.