I swear assertiveness training should be part of prenatal classes! Not even joking!
Op as a parent you really are going to have to learn how to assert yourself (and your dp does too) because this is VERY much part of parenting, not just with grandparents but with drs, midwives, hv, teachers...
As women we are trained not to "upset" people and this too often means subjugating OUR needs.
I say HELL NO!
My in laws were lovely - it's MY parents that are the nightmares!
With people who trample all over your boundaries and WON'T take a hint you have to be civil but blunt!
Learn NOT to use the word sorry in almost every sentence (seriously I notice this a lot in women), be clear, concise and direct.
That's not rude, it's assertive boundary setting.
And prep dp to have your back!
I think you would HUGELY benefit from learning about toxic families and FOG etc
Not read myself (probably should!) toxic in laws by I think Susan forward?
Also check out this website
https://outofthefog.website
Because it very much sounds like your mil may have narcissistic tendencies - using a history of illness to control others and as a threat to avoid being "upset" is a classic tactic! Both my parents use that one!
All that said re buying things for baby that stay at her house - as pps said it's their money to waste BUT you can also find it very useful especially when baby's very small not having to either keep carrying the baby or lug stuff to theirs each time.
My parents also had a travel cot, microwave steriliser and spare bottles and dummies, changing mat and spare nappies/wipes/bum cream, spare baby/toddler plates, spoons etc at theirs plus a small box of toys and books for the doc which just lived under the stairs when the dgc weren't there and it did make it much easier to visit them.
What was odd was they did all this just when I had dd and at that point bro was saying he'd remain child free and sis thought she couldn't conceive (although was starting down route of getting help for that) so only 1 dgc so it was slightly bonkers...
Then bro got his 1st wife pregnant unplanned (they weren't even engaged at this point) with his 1st, he's since had another with 2nd wife too, sis got pregnant via ivf, and then got pregnant again naturally twice (it was like the ivf triggered her own stuff to work finally, although not sure of impact of it being a different dad for younger 2) so mum went from 1 dgc she thought she was lucky to get (I've had issues too) to 6!! 
Find your inner and channel them in your interactions.
Good luck 