This issue has been building up since I had my dc1 19 months ago. What started off as anger on my part has turned to worry that there is something wrong with MIl.
I've been married for 15 years and obviously known MIl all that time but we had children late (thirties). Second child now due.
Before kids, my MIL and I got on well, we enjoyed meeting for coffee, having a glass of wine, shopping and I loved her coming to stay overnight, we had lots to chat about and I genuinely enjoyed her company.
Fast forward to now and I think her personality has changed. She is very repetitive, forgetful, quite lethargic and doesn't really seem bothered about talking to me the way she used to.
Anyway she lives an hour away so I probably wouldn't have noticed this to be honest as now I have kids and work full time I don't really see her that much.
BUT, she comes over to care for my dc around 2 days a month as my childminder can only take him 4 days a week so we are occasionally stuck for care.
I've been really cross about some of the things she's done with dc1 lots of minor things but it's all come to a head recently. I can't understand the change in her behaviour, deliberately undermining dh and I, she's not a bad person, she's a (was) a lovely person so I feeling there must be something wrong with her.
I don't want to list the specifics of things that I've noticed but some examples are; going on walks for hours and not coming back until well after dark, when dc should have had his tea and been in bed, giving him milk that was clearly too not, leaving dc dressed in his outside clothes to nap in winter (heavy quilted all in one suit) when he comes back in and forgetting that he was in a room when the heating was on- I came home and he was sweating profusely and was listless and spaced out for ages afterwards. 
I honestly could write a book of examples but basically I am concerned about her ability to care for my child properly and I don't have confidence when I go out to work leaving him.
She will be very angry if I told her she's not looking after dc while I'm at work anymore. I would just feel happier with her in the role of granny and not having sole charge of my child.
She in her seventies and has to leave very early in the morning to get to us.
She also taken this idea of taking dc out in her car which I'm very uncomfortable with.
AIBU and if I'm not what can I do that won't upset her.
I've also found that when I'm talking to her or trying to explain things she seems a bit 'vacant' and doesn't really process what I saying.
BTW her children have noticed something isn't quite right either but they have grown up children.
I don't want to upset the family.