Sadly, there is no law for managing such kinds of cases, you have to deal with this type of situation out of court. This family conflict depends on why your maternal grandparents are behaving this way. You being a child are not allowed to ask questions to your elders when they do wrong something wrong. But, on the same hand, you also have to be brave enough to fight for your rights. You must talk to your dad about this and let him know what do you feel.
Support your dad and be with him, ask him to stand for himself. Encourage him to take steps against this kind of injustice. There is no one else who can help your dad rather than your dad himself. Make him realise his rights and support him to win back his respect.