What can i do if someone calls DCF multiple times and was unfounded?

My mother has custody of my sisters 5 kids. Someone keeps calling DCF on her. Dcf has been to her house 3 times this year. All of the complaints were unfounded and were dropped. Dcf came out once again today. I think that it may be the other grandmother trying to get custody of the kids. DCF will not tell who has been calling. How does she find out who is calling and get them to stop?