As a Step MIL can I give my take?
I never ever ever wanted children
I met a man who had a 2 and 3 year old , they lived 500 miles away so my only interaction was holidays - where they would spent a week or two with us - they have only ever known me and their dad together
So now I have 3 grandchildren - 4 to 19 - same parent - 2 fathers - me and granddad have been their constant
DGC 1 - I raised for the first two years of his life - never in my life having had a child of that age in my care in my life before
I made mistakes
But I love that child and I wouldn't put any child in harms way
DGC2 , DH wouldn't have them with us over night till they were 2 - because of the way they had been bought up and our age - we were a lot older and coping with a toddler was looking to be too much - and it was hard the first few times we had over nighters
DG3 we have over night since aged 2 - because we have DGC 2 stay as well
Our grandchildren love coming to us, well the 19 year old has backed off a bit as he is working, has, a GF etc
We offer a different experience to what they have as "normal" and some things we might clash on but mostly the children are happy to accept the rules - meals at tables , some sort of fruit or veg has to be eaten, bed time is bedtime and tablets are taken away
We cherish the time we have together without the parents. There's no clash about what the rules are - they are here and its our rules, and we are perhaps a bit stricter then their parents , but there's not a lot of kick back
Your MIL can actually bring a lot to a childs development so dont say forever it's not happening. Yes I was knackered having to put the needs of a small child baby before my own, but it came natural - even as a non birth mother
Your MIL will look after that child if it was the most precious possion - which it is