To not want to regularly look after my SIL DC

[namechange201841]

Trying to make this objective as possible. I have a fairly good relationship with SIL. She is a single parent so I know it must be hard. I currently work Monday to Thursday with Friday off I have two DC one at school one at nursery. I use Friday to spend one on one time with my youngest and also try and get as many job done around the house is possible.
SIL is going back to work soon and recently announced a great idea for me and MIL to get a better bond with her DC. MIL would have DC every Thursday and I could have him every Friday. It was sort of asked but almost worded as if she was doing us the favour.
MIL has afew health conditions and told her she couldn't commit. I said similar that there was no way I could commit.
SIL came back with a new plan of me and MIL just having alternative Fridays. MIL agreed but I quite simply do not want to. AIBU?
If I'm honest some weeks it might not be an issue but I don't want another commitment. Some Fridays I'm shattered (my job can be stressful).
I have looked after SIL DC on a number of occasions and do see them about once every two weeks. In an emergency or occasional favour that is no issue.
A side note I get it is to save her some child care costs, however, she is on UC and would get 85% paid

Tell her she can have yours every other Saturday in return, she will soon realise how unreasonable she is being

but happy to meet up together with the kids at weekends’

I'd be careful about saying this as you'll be opening yourself up for more and more of your precious time and energy. 🌹

Agreed, OP I think you need to be clear that you're tapped out with your own DC and job.

'No can do. My Fridays are already accounted for. Do you need help finding a local childminder?'

Cheeky cow.

Perfect.

Absolutely no with knobs on. Why do you need to 'bond' with her child? No good can come of this.

Very presumptuous of her

Your day off is precious and it is hard to work 4 days and have your own dc to look after

Yanbu

Take it off the group chat and tell her you can't help. I wouldn't offer excuses or give reasons per se as it just opens negotiation but I think you can say something along the lines of 'I'm already just about coping, it would be too much for me'.....although cheeky fuckers often don't care about you and your life / issues / needs.

Make sure DH is onside even if he isn't involved.

No I wouldn't even if I was paid. Very cheeky of her to put it forward as if it's some benefit for you.

This is excellent.

Don't give her an opening for weekends or holidays. Stay as arm's length as possible.

She chose to have the child, apparently with a loser for the father. She'll have to manage the consequences without infringing on others' lifestyles.

SIL is going back to work soon and recently announced a great idea for me and MIL to get a better bond with her DC. MIL would have DC every Thursday and I could have him every Friday. It was sort of asked but almost worded as if she was doing us the favour

Erm, that'll be a no for me! How cheeky of her. Just say your Fridays are already accounted for, she doesn't need to know what. Maybe say if in an emergency you can look after her dc, but not as a regular thing.

Wouldn’t offer ‘up in an emergency’ as there’ll always be an emergency - poorly mother, childminder away, can’t get to nursery, going into work early/late etc.