Here's an idea get your mum to visit at the same time as in the in-laws and have her say something like I better leave you guys to it, I don't want to intrude on any more of your time with DS and hopefully MIL will take the hint
I'd go further than this if your mum is up to it. 'Right then, that's 30 minutes, that's the maximum the hospital said, wasn't it? Come on, MIL, let's take our cups through to the kitchen and then leave these good people to get some rest. Have you got your coat? Oh look, you've left your scarf in the corner of that chair, don't forget it. Onthedowns, we'll see you soon, do you have any ironing or washing I can take with me?'
And get her to physically bustle your MIL out of the door.
I have two sons and if the time comes I plan to follow my mum's example of how to be a good MIL. She is thoughtful, considerate and helpful. She doesn't assume her solutions are the best, and I often have to drag advice or opinions out of her which is occasionally annoying -- but she is so worried about being overbearing having had an overbearing MIL and mother.
I have two SILs. One has a normal, sane family and gets on well with mine. No kids. My other SIL has two small kids and a self-involved, attention-seeking mother who never offers help, complains that she doesn't have a close relationship with her grandchildren but won't put any effort into building one, expects to be waited on hand and foot and generally goes out of her way to make SIL feel like shit.
Curiously, it's my mum - her MIL - with whom SIL has a much better relationship and who has a much closer relationship with the grandchildren.