I remember when my sister was pregnant and we were discussing what she would do after mat leave. She has a well-paid job. At the time I'd more or less decided that I wouldn't be going back to work after my mat leave with my own DS because I was only in a very low-paid job with not much prospects and childcare wouldn't be affordable. My mum had offered to mind my DS part-time but it was too far a distance between her house and my work and the petrol money and hassle of getting up at the crack of dawn just wasn't worth it. I knew we would struggle with money on just DH's wage but on balance I considered that it was doable for a year or 2 and would mean that my DS would have my undivided attention.
upon hearing I would not be returning to work she just assumed that I would look after her kid when she went back too. For nowt. I was pretty shocked at that assumption. I mean, 6 month old babies are hard work and I had my own to look after. I just shrugged and said "we could try it. You'd have to pay me a bit. Not as much as nurseries obviously. But I would need money to cover expenses at the very least."
"But why?!"
"To be honest if I was choosing to look after a second baby then I may as well become a registered childminder and earn some money for the extra work it would bring as DH and I struggle on one wage."
She really took umbrage that I would want to charge even a small amount for the task of looking after her child, while she went back to work guilt-free, knowing that her beloved child was being looked after by a blood relative, not a nursery, earning a tidy sum of money in the process, all of which she would be able to spend on luxuries like designer clothes and holidays, while WE were struggling to pay the bills and having the added hassle of minding her (difficult) child.
I still think all these years later she feels I was wrong to refuse. I don't think she has any idea how cheeky I thought her request (assumption) was.
I am perfectly generous with childcare. I don't work in term time and I mind her now school age kids a day here and a day there and have taken them to and from holiday clubs.
But a demanding baby, for free, is a different matter, when I had my own!
She quite often thinks the world revolves round her and her wishes though and quite often fails to see someone else's point of view if it differs from hers.
OP, you need to stay assertive and if it comes to it when asked why not just say "I don't want to, it's a hassle and reduces the choices of how I spend the day with my own child."