My MIL used to do this with Ds1. She tried it Ds2 and I just walked up and got him and said you can have him back when he calms down. or Dh would do it.. I said to Dh you really need to help me out when your mum is like this and he would.. He just didn't realise how hard it was for me..

Anyways I hope your MIL stops it soon because it is distressing listening to our children cry for us and know we can stop them at any time..

Honestly though just be firm and say I will settle him down and then you can have him for a cuddle. Maybe explain before you pass him to her that Ds gets a little upset when Mum and Dad aren't there and that you need to comfort him when he is like that..