Why are you doing one day a week?
I mean that's your choice, but if you think that giving them one day a week will stop this, it won't. They will believe they can, eventually get you to do the 4 and won't stop asking.
Op, my mum and dad are I a similar situation. Dbro and his wife really expect a huge amount out of them. My parents did 2 days a week for me with my first. When my second was born, 7 years later mum told me it was too much to have 2 regularly. So we got him child care. Her health had declined during this time. It was absolutely understandable. Another 5 years later, after mum's health got even worse dbro and his wife had a baby and were so angry mum wouldn't look after the child. They felt she had done it for one of mine, she should do it for one of theirs. They didn't accept that she had aged and become ill. Worse still one of them was going to be sahp. but they still wanted regular childcare.
Mum and Dad did, however, do all emergency childcare. Babysat so they could go out or sil could go meet friends for lunch, take the kids away for the weekend. But for dbro and sil it's never good enough. If mum can't change plans they threaten to cut her off. If dad can't change plans they say very little. My parents have also, given them money when they were struggling paid for but of work for their house that needed doing. Gave them their second car, taxed and insured it for a year. My parents do loads for them.
My dad is continously, stepping in and telling them both they are being awful. They back down, then do it all again.
Mum has ended up having to put hard boundaries in place.
As in your situation its often led by sil, with dbro going along with what she wants. But I often think dbro likes it looking that way as dsil gets the blame. In my opinion my dbro (and your son) are as much to blame. They aren't children. They are adults making a decision to treat their parent badly.
Also like you, sils mum is amazing and wonderful etc. Yet has never looked after their child or helped them out in anyway. Not with anything.
Its heart breaking to see my mum being treated like this. But, both mum and dad let them get away with it for too long so now that's the dynamic.