You sound so lovely with so much on your plate.
You would be out of your mind to take this on.
This is not about being honoured.
This is about free childcare and you being seen as a soft touch.
Your SIL wouldn't dream of doing it for YOUR children and good on her.
Providing free childcare is a huge imposition and one a lot of women balk at providing for their own children, such is the enormity of how restrictive it is.
It is absolutely thankless too.
I wouldn't dream of involving myself in my wider family's childcare needs.
Your niece and husband need to find a better paying jobs and rely on THEIR parents more.
This is nothing to do with you.
Suggest your DP for the role🤨.
I would hesitate to even mention emergencies.
You have your own family and an infirm 90 year old father.
Your niece is a CF as is her mother.
They know well you have responsibilities but don't care.
What if a job comes up that suits you but conflicts with you providing free childcare that involves you driving to their house?🙄
You will undoubtedly be guilted for "letting them down"....
I may sound harsh but you sound too nice and too easily put upon by your family of presumptuous CF's.
Be firm on your NO.
You will bitterly regret giving an inch to family who will privately think you are a mug and wouldn't in a month of Sundays do the same for you.
Its a great trick when asked a BIG favour that you feel hesitant about, would they help me in such a way?
If not, then absolutely answer NO
Free childcare is a HUGE ask IMO.