It sounds like your DP is getting a bit wrapped up in your immediate family unit and not really appreciating the joy it would bring to a 90 year old to meet her great-grandchild (and step GGC) and see her grandson happy with a partner. I think that we all have a tendency to dehumanise old people, especially ones we don't know. If it is the case, can you talk to her about what your Nan means to you, maybe show her pictures of you and her when you were a little boy, that sort of thing (nb this might be a bit difficult for her to swallow if, like many of us, you've been lax about visiting her for many years, but it seems to me that having your DS has maybe made you realise it's time to make up for that).
As other posters have said, at 90 she may not be around much longer. My Granny turned 90 last September, was in relatively stable health, but she took ill and died last month. It happens, sadly. It's also worth remembering that when you are 90 life can be pretty dull and hard going - reduced mobility, too deaf for the telly, too blind to read comfortably, so getting the chance to meet the baby will be a huge novelty for her. (unless of course she has a million grandkids and loads of other DGGs and is already getting them all mixed up
.) And let's face it, we can sometimes have trouble finding much to talk about with the elderly if they don't really understand or identify with our jobs, social lives etc so the baby will be a lovely thing to bond over.
If your DP has a grandma of her own, perhaps ask her to think of it that way. If she has no grandparents, perhaps remind her that that is why she is maybe not realising the importance of the visist?
If it's really the big party occasion that DP is nervous about, can you maybe arrange for just you, DP and the 2 kids to visit another time? Your nan will be getting lots of attention at the party anyway, maybe she'd prefer to have a separate visit to look forward to once it's all over. You could send a big photo down in a card with your Dad, apologise that you can't be there (just make up an excuse), say you're all really looking forward to seeing/meeting her and set a firm date.
It does seem that there is no real physical barrier to DP and the baby making the journey, so maybe appealing to her better nature, in a gentle way, is the way to go. Let us know!