Keeping relations with a family member who turned away from Islam

My sister in law has converted to the Hindu religion by marrying a Hindu boy. She is living in a Hindu family and following Hindu rituals. I know it is a great sin to leave Islam, but my mother in law and brother in laws are not breaking their relationship with her. Due to this she usually comes home to meet them. In this case what should I do? Is it permissible to keep relations with this type of a person?