IMO it is unreasonable of any parent/parent in law to think it is their right to have a grandchild in their care on their terms. It is the sole responsibility and decision of the parents to decide who looks after their child.
Grandparents had their time of raising a family and presumably raised capable adults who now have the right to dictate how their children are raised and cared for.
If OP's MIL is terminally ill then one has to ask if she is fit to have the charge of an 8 month old baby.
Being quizzed by chemist's assistant in Boots.
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