My wife is begining to bow to the pressure of her mother and is now considering bring the child to grandma's and not tell her sister, who is vehemetly against it.
You can guarantee the sister will find out - 4 year old child excited to see mummy after a little holiday and will tell her all about everything she did, including the lady we went to see who said she was Grandma !
I hope you can talk you wife out of it as it will hurt her in the long run