Don't want SDIL to use "special" name - unreasonable?

[CultOfSkaro]

Background: Been with DP 11 years, he has 4 DCs, all with his exW but one is a few years older than the others. By the time I was introduced into the DCs lives, oldest one had a FT job, got engaged and was living his own life so I have never been in a vaguely parental role with him.

I am very close to the other DCs and a few years back, they came up with a "special" name for me - it's a vague derivation of my real name, which no one else uses, and recognises that I am a StepMum (even though we never got round to the marriage bit!) without using the word Mum, as they have a fantastic Mum already. I always refer to them as DSS / DSD as I don't think a marriage certificate makes our relationship any different.

Oldest DSS's fiance (SDIL) decided over Christmas that she would also start using the "special" name - even though DSS doesn't. She said she "wanted to follow the trend". I feel really uncomfortable with this, as does DP. The name the others came up with recognises that I have looked after them when they are sick, read them bedtime stories, cuddled them when they are sad etc. I've never had a close relationship with SDIL, she has made a number of comments over the years about it being unnatural for women not to have children (I can't), and used to pointedly remove the grandsons from my presence when they were small if I was left alone with them. OTOH, perhaps she is trying to make up for all of this by using the name, which makes me feel like a bitch if I contemplate telling her I don't want her to call me that. Can't work out if I am being unreasonable or not.

Can't quite understand why in the context of your relationship with her she wants to use that name in the first place? From your thread it sounds as though she is a grown woman with children of her own.
Two scenarios here:

1. She knows you are uncomfortable with it and is doing it to wind you up

1. She knows that you are not close and would like the relationship to be more so, is making an effort to have the same kind of relationship as you do the other kids.

In the first I would just ask her to use your christian name from now on and frame it in a way that you view her as a friend and would like for her to call you the name that other friends use.

If you feel that it may be the 2nd scenario then I would try and go with it and aim to develop a closer relationship. Forget what has gone on in the past and try and move forward (easier said than done I know!)