mom of two,
I think you need to observe them together more closely. What type of personality is your mother in law. Does she like children? Does she have a pet that she brings with her. Does she stop over? Does she get up in the night with your daughter? Does she come to stay on a regular bases? Does she go tell your daughter good night? If you are absolutely positive that the reason your daughter is up screaming in the night is due to your mother in law's visits then you must observe them to find out what the problem is. Does mother in law feed your child while she is there? Could it be that she feeds her things she isn't used to like too much sugar, too many sweets, and sweet drinks? Does she over indulge your daughter?