On again/off again girlfriend/kind of step mum.

I feel for you. Now she's got her own child no one will match up I'm afraid. How does your son feel about this.

This is down to your son to deal with. If it is having a big affect on your Grandaughter, maybe the Father can see her elsewhere, and bring her little brother with him.
I would not get involved and leave it for him to deal with.
Your son needs to speak to this woman about her attitude towards his daughter, belittling her is not acceptable.

I'm not sure he even sees it. He's the defensive type. It's not a stable relationship, this is possibly their 4th try.

You need to find a way to make him see it E173 so that he can talk to the mother of his son. Of course the feelings she has for her son will be completely different to those for your son's D, but that's no reason to belittle a 5 year old.

If he doesn’t see it then how do you know about it? Are you present at every visit? Sorry for all the questions but do they live together where is the child's mother? Is the child upset about it?

I know about it because he brings them here on the weekends he has the children. He currently doesn't live with his sons mother either (again, long story but they've had 2 goes at that too). He has BPD and isn't suited to permanent relationships. This weekend she showed up out of the blue for the day. Same old story.
I get on well with gd's mum and her partner.

P.S. I did think for a while perhaps I was being over sensitive until my mum asked had I noticed that gs' mum ignores gd or snaps at her.

I would feel compelled to defend a young granddaughter (or any child) - so I'd challenge the girlfriend directly by pointing out her unacceptable attitude and behaviour. How would she feel if her son were treated that way?

Hetty, that will happen and I'll just have to deal with the fireworks, I think.
Whilst they were apart the last few months I was hoping it was permanent this time because that way gd doesn't see the girlfriend at all.