Sorry, this might be long but it's all relevant so bear with me......
My dad and dsm have been together since 1990. Dsm and her then-dh were friends of my parents, dad and dsm had an affair and set up home together and eventually married once both divorces were finalised.
Obviously my mum was devastated by the affair and my dad leaving but she met her dp in 1992 and they've been together ever since.
I was 13 at the time, my dsis was 12 and my mum said and did a lot of things (cried when dsis and I were supposed to be going over to dad's for the night, emotional blackmail stuff) to try and wreck my relationship with my dad. I didn't speak to him for nearly 2 years, didn't go to his wedding and had a very rocky relationship with him until I was 19, went to Uni and basically was able to realise what she was doing. Now I'm very close to both of them and they have a very good relationship with dh too.
Fast forward to 2003 when I was expecting ds1. Mum told me that her dp would be known to my dcs by his name. That was fine. It was her choice - we were happy to go along with what she wanted.
However, she also stated that she thought dsm should also be known by her name, not Grandma/Granny/Nanny, etc. Her reasoning for this was that my paternal grandad left my grandma for another woman shortly after my parent's wedding and my dsis and I called his new woman by name.
DH and I didn't agree with her, because dsm had been with dad and in my life for as long as she hadn't, iyswim. However, we didn't make an issue of it, dsm became Grandma, as that's what her son's dcs called her and that was that.
We never actually explicitly told her this, which was probably stupid, but we assumed that the dcs would have said it as she often asks them about what they've done with my dad (he lives in Singapore and we go over for a fortnight every year).
Jump on again to the present. My dsis' baby is being christened next month and both sets of parents will be in the same room for the first time. Dsis asked if mum knew about the dsm/Grandma situation and I said I didn't know. So she asked her and it turns out that she had no idea. I am genuinely surprised that she didn't know but there we go.
However, mum has gone slightly off the deep end about it. Not to me though, to my dad via email.
So how do I approach it with her? My instinct is to tell her it's actually none of her business, she made the decision about her and her dp's names, this one was down to us and she has to like it or lump it. It all happened 18 years ago, she's been with her new dp for 16 years, we've all moved on so she needs to get over it.
So I need some MN wisdom because I suspect that's not the best approach . What do I do or say? I'm ringing her in an hour or so to discuss it.
Help!