I posted about something similar to this in Jan/Feb this year.
I had begun to look after my granddaughter because my daughter was returning to work.
I hadn't seen her much because of lockdown/childcare bubbles. I was effectively a stranger to her. Boris Johnson had decided from 2nd December that those with children under 1 yr could have some support - so we visited as much as possible from this date to build a relationship with our granddaughter.
To begin with I had her one day a week. The first week she was very quiet. The second week she started screaming as soon as mum went out. She was still screaming half an hour later when mum arrived at work. I rang mum and was told to take her for a walk in the pushchair - it was raining. I walked about for three hours, she had about 30 min sleep in between sobs/screams.
She started screaming again as I tried to put her in the high chair for lunch. She didn't eat any lunch. She was screaming/kicking/wrestling when I changed her nappy.
She was so upset - I rang mum and she came home early from work.
Granddaughter was upset. I was upset. Mum was upset.
My ears were still ringing when granddaughter finally stopped screaming.
It took another three weeks before we had a day without screaming. I was conscious of others thinking I was mistreating my granddaughter - when I was struggling to get her into the pushchair when her body was rigid and I was trying to get her legs under the bar and her arms into the harness.
We can still have 'episodes' of screaming when I take her out of the swing in the park and try to put her back in the pushchair. Or she doesn't want lunch at lunchtime. Or she doesn't want a nappy on/changing. Or she doesn't want to be in a shop/pushchair/car seat. It's been a difficult and fine line to follow - she's a strong willed little girl and 'going along with her wishes' could be seen as 'pandering' to every whim. But I'm desperate to keep the peace and not induce screaming as it is so upsetting for all concerned.
I did not allow this with my own children they followed my routine - but maybe they were more easy going children. They did not dictate how the day was going to pass. My granddaughter has me completely wrapped around her little finger - and I let her get away with it.
I absolutely adore her, she is funny and inquisitive and quite advanced with regards to milestones. She certainly knows how to get what she wants.
I just thought this may be another perspective for OP to consider if her child wasn't used to being left with MIL.