Advice Please

What Bluebelle said. It's not your fault. Your son should have asked if she could come too.

Yes, just speak to them. It was really up to your son to ask why you weren’t inviting her. That’s why you would have overlooked it. Don’t make it bigger than it is.

Why are you posting the same thread under two names?

Invite them around and get a bunch of flowers maybe. We are all probably feeling a bit sensitive it's been a difficult time. Hope it goes well for you life is too short to fall out. ?

I hope the call goes well, Nannysprout.
It's clear that you didn't see it as a get-together and your son had no idea it wouldn't be a visit for him only, as usual.
Why not invite your son's partner to come with him for future visits, now the rules have changed.

Thank you very much for your replies they are much appreciated. Just to explain a little more my son had been visiting on Sunday afternoons on his own because of restrictions and at the same time his partner had been seeing her Mum on her own. He didn't know his sister and family would be there on this occasion as it was a last minute rearrangement. I was so hung up on my daughter forgetting our first arrangement I didn't really think about telling him they would be there. It really was just an informal meet up not what I would think of as a proper family get together but looking at it from my sons partners point of view I can understand why should would have felt slighted. As they have spent many long months in lockdown together in a small flat both working from home I also thought even if she wasn't seeing her family she would have been glad of a little bit of space for a while. To clarify though I had no idea as to whether she was with her Mum or alone and I feel bad that I made an assumption that she wouldn't have wanted to be with us anyway. Its all a long story really so I appreciate it doesn't make sense I was just trying to keep it short. However thank you for your advice and I think I will call her and apologise.

Well I don’t understand why your son didn’t ask for her to come along or say ‘mum you ve not mentioned x coming with me’ Why would he leave here at home and come on his own I don’t understand why it’s your fault
Sounds very peculiar

Text is no substitute for a face to face or a phone call. You need to explain your oversight but, I ask, did you not ask your son about her at all?.Doing so might have spawned a conversation about her being at he alone. Why did your son, presumably knowing his wife was not going to family, not ask if he could bring her to yours too. All seems a bit odd but whatever the background, in your shoes I'd visit her & explain.
Next time, make sure she's definitely on the guest list. It'll then be her choice whether she comes over.

Just ring her and explain what happened in the same way you put it in your GN post. I think she will understand that you didn't intentionally leave her out. Hope all goes well for you.

Pick up the phone, speak ear to ear, best wishes