Our LO has never "gone to" MIL. She lives locally and has seen him a minimum of 1x/wk. since he was born. I am positive and pleasant towards her and around her. DS2 is 17 months and, while he will give her a smile and play around her, he won't go to her (ie: let her cuddle him or pick him up). I think it's pretty age-appropriate, plus I'm a SAHM and he's pretty bonded with me. Well, last week she'd kind of had it with his not going to her and she said "Oh, I see...you're a mama's boy." I didn't like it and told DH later that our son had been called his first name. He wasn't too bugged about it since he knows his mom and he also lets things roll of him pretty easily. I do understand that MIL's feelings are hurt.
He does let other folks pick him up and play with him and stuff. He's very happy and social, and no, I'm not worried at all about his social development.
If she says it again, what should I say?