Grandmother often refers to herself as "Mom"

mcem - yes, I've been call 'Mum' by pupils (who then look very embarrassed and giggle) but it was when I started getting 'gran' that I knew it was time to move on!

I call my grandchildren a range of names (across the generations) until I settle on the correct ones.

MIL sounds a tad barmy if she refers to herself as Mummy to het grandchildren......is she a bit?

I wouldn't like that at all and would need to say something especially to the kids
However my poor grandkids go though a plethora of names before I get to the right one as my grandaughter said to her cousin just recently after I had called my grandaughter my other grandaughters mums name don't worry I m used to it

What I'm not sure about is what they call their mum / mummy (usual UK, although mam & mom are known)

I knew a number of UK children with American fathers who called their grandmothers 'grams' (common in US I
gather) but a couple who said 'Mommy' like their father, and saw that as a separate name to mum / mam (both used where I grew up)

If she is regularly and deliberately trying to call herself their mother then something needs saying. If she is using one vowel to distinguish herself from Mummy that's a different case - does she ever use another name?

My grandchildren quite often call me 'mum', they usually correct themselves within a nano-second.... if not I correct them ! Although they never make the mistake when mummy is there, only when they're on their own with me.There is no excuse for a grandmother to do it unless to annoy.!! Tell her she's losing her marbles. That should do it. She should have pride in being a grandma. I have.

If it seems deliberate, dh needs to talk to her. It may mean more coming from him.

I've always been Grandma but occasionally a GC will slip up and call me Mum and then giggle.
My husband calls me Mum and always when my children are around (38 40) and I call him Dad when my kids are around and when referring to him. Otherwise he calls me love, sweet etc.
My Dad is 92 and called his Grandma: Mother and his Mum, Mum.
It may be a generational thing or which area of country you are from.
Personally it wouldn't have bothered me but you can always correct it by repeating whatever she says to your children with Grandma inserted instead of Mommy. i.e Grandma just asked you to come to the table. Etc. Then it's not too obvious.

we stuck with first names in our family. So no mummies/grannies/grandpas/dads. But our DC knew who was what and who loved them.

My dd just told me that I used to refer to myself as mummy when her twins were small and we were all exhausted with feeds every two hours. She assumed it was a slip of the tongue and I was not aware of it until now. Oops

When my first grandchild was born, I was so excited and so madly in love that I know I went over the top. I must have annoyed my lovely DIL rotten with my behaviour. I bought presents left right and centre and took every opportunity to get my hands on the baby. I got told very gently to lay off.

All that I did was through the sheer excitement and flood af absolute love I felt.
I am so glad that I was treated with such understanding and kindness instead of suspicion and unkindness.
Now I have several grandchildren the excitement has somewhat worn off. So please give your MIL a break. Surely the more people who love your children and are in their lives the better.
You know that you are their Mother and not your MIL...so relax about her slip of the tongue as I am sure she is not plotting to undermine you and maybe you could be a little bit glad that she takes such an interest in them and wants to be with them.