Oh dear. I can see this from both sides. As a grandma, it’s lovely to spend time with grandchildren but you, as a still relatively new mum to your DS, shouldn’t be doing this heavy lifting. And it does sound like heavy lifting, emotionally, physically, psychologically.
You don’t mention your own parents or wider family in this. So, I’m going to give you the advice I gave my own kids when their children were born and, no, I’m not your mum, but please take the advice in the spirit it is given.
When family want to come and spend time with your children, they are just that. Family. They aren’t guests in your house who must be waited on hand and foot and everything be perfect. They are family and family means that if they want to come round, they can pitch in too. Dinner needs cooking? Delegate. Laundry piling up? Delegate. I’ve seen many grandparents who are afraid to speak up and say “I’ll cook/wash/clean/vacuum”, in case their DIL/SIL gets offended. But you can and must speak up. Saying you need help isn’t a sign of not managing well, or being a bad parent, in fact, it’s the opposite! It’s showing that you are a good parent; you know when you require assistance and you’re not afraid to say.
I love cleaning. It’s therapeutic and gives me time to think. What is better than organising cupboards or pantries and folding clean washing? I admit that I’ve gone to friends houses and cleaned up. Not because I think they're dirty or because I’m judging them, but because I can see they are busy people, with busy lives and many hands make light work. They don’t get offended but understand that if I’m sitting in the living room, talking to them, I’ll tidy up. I think a lot of it too is my ADHD. I can’t just sit still and do nothing. I like have to keep my hands and mind busy.
When grandma visits, this is your time. Need a haircut? Leave the kids with grandma. Need a manicure? Grandma is there. Need to go shopping, just to get out of the house? Bye grandma. Your MIL isn’t some delicate little flower whose petals will drop off if you leave her. She’s a competent lady, who lives alone, yes, she sounds lonely, but she’s also emotionally blackmailed her whole family in to looking after her! Well, no. You are not her mother! She is yours (or rather DH’s so by virtue of that fact, yours).
It sounds like she’s bonded beautifully with DD but I’m not seeing where she’s bonded with DS. So, next time she comes round, make sure all the bottles are prepared for DS, the nappies and a change of clothes can be found easily, if/when needed, give her DS (if he’s awake) and say “cheerio grandma, I’ll be back later. The chicken needs putting in the oven at 4pm at 180C or whatever if I’m not back in time/turn the slow cooker off/put the stew on to warm (whatever you have planned)” and then leave.
If you make plans, you know she won’t get there before a certain time, so text her saying you’re going out and won’t be back until xx time (after it’s got dark, which is probably really early right now) and could you please leave it this week as you have a lot on. And then leave the house! If DH doesn’t like it, he can take the day off work to entertain her whilst you and the kids go out.
When she’s there, make sure she knows she’s there to help. “Grandma, could you quickly vacuum whilst I’m cleaning the bathroom”. “Grandma could you do this load of laundry while I’m making the dinner”. “Grandma can you hold DS while I do this”.
The biggest problem you have is that you didn’t start as you meant to go on. She’s used to being an honoured guest, not a family member. You need to tell her where the linens for her bed are. She doesn’t make the bed up? Not your problem. Also, you need to get rid of DD’s bed and think about getting bunk beds, as I know it’s a little way off but eventually, your 2 will need to share. And you should think about putting DS in with DD in his cot and using a monitor for when he cries at night (assuming he’s not sleeping through). DD will sleep through his crying, but I bet grandma won’t! Let her assume responsibility during the nights so you can get some rest.
Remember, grandma is not a guest! She’s family, so treat her as such 🌹