I also face this problem b4.
at first, I insist to keep a distance from mil. n I spend more time on my child, talk n play with him. then he stay closer with u, next time he go mil home, he will say no(that's my son haha) everytime mil ask him stay longer or stay overnight, I will emphasize to him: I will not stay with u ah , mama go home sleep. u stay with here only. he cries n says NO to mil. then mil will snatch him from my hand, my son will feel irritated hahahaha :)
the most important that is spend time with them, they can feel. u r his mother, he loves you more than others
Mum To Be