Thank you everyone for your helpful, kind and supportive replies.
I did briefly think of going to visit DD and the DGC when LB is attending his sister's wedding, but dismissed the idea. I fear it would make me seem 'stalkerish', needy and desperate, as well as a bit sneaky, and as Tribpot said, it would be bound to backfire on me. DD would, I feel, be alarmed if I turned up because she would be afraid of getting into trouble with LB when he got back. She would not want me to stay and I would be devastated. It would also be unfair on the DGC.
I said I was sorry to DD because I genuinely was. She was so excited about my Christmas visit and that LB and I seemed to be getting on better. For him it was a really big thing that he was prepared to eat out in a restaurant, as he never likes to eat out, but also that he was prepared to go with me. I know that DD had a happy feeling that we were going to be as a family and it was also a really big thing for her, because she never goes anywhere either. For my part, I was ambivalent because I always feel tense around LB, everything is on his terms, and DD tiptoes around him all of the time. It was also a long way to go, in the freezing cold, on my own, with two full suitcases of items which could mostly have been bought in Ireland.
However, I was and am very sorry to have hurt her. I spoke to her the following day and, but for LB, we would have sorted it out and been reconciled. But I knew that she would talk to him, of course she would - who else is there to talk to? - and that would be the end of the relationship, which we were painstakingly rebuilding, for the foreseeable future. The relationship has been so precarious and fragile that, as Billybagpuss wisely commented, I was constantly worrying about saying or writing the wrong thing so, in a way, the 'famine' is less stressful.
Oompaloopa - I don't know LB's family background well enough to know if there is any history of trauma/MH/PD, only that DD told me that LB's father and grandfather are like him in that they would happily never see anyone. LB's mother has been pleasant and welcoming and sent flowers when DH died, so I prefer just to take it at face value. I think that they are shell shocked by LB and DD's recent treatment of them. DD also thinks that LB's sister is the 'golden child' and I suspect that part of him is enjoying being able to punish them, by withholding their DGC, in retaliation for perceived past slights.
I was talking things through with my new therapist this week, which was helpful, as she made me feel that it was understandable that I became so upset before Christmas. Actually, being excluded from the baptism was quite triggering of the earlier rejections and, although I managed to dampen down the hurt I felt at, once again, being discarded, the rejection of the baptism gifts was the proverbial straw. My therapist made me see how the relationship was never secure and that is why it has been so difficult for me. The phrases I kept saying to DD were 'everything is thrown back in my face, why do I bother' and that I was 'surplus to requirements', which is how they make me feel.
I was telling my therapist about how LB and DD sent two cows to the abattoir because they were 'grumpy' and she asked me if that was how I felt, like the cows. And I realised that she was spot on, that I do feel like a grumpy old cow that is not wanted any more, and who has been despatched to a mental and emotional form of abattoir. I said yes, that's exactly how I do feel, like Boxer in Animal Farm, that I have outlived my usefulness and would be worth more to them if I died and they could secure an inheritance. I don't actually feel that DD wishes me dead, but I have no doubt that LB would be very happy at my demise. However, unlike Boxer, I do have autonomy and financial independence, so I am not powerless the same way.
In fact, when I was in the 'feast' period, it was really more a case of DD being dependent on me, rather than the other way round, at least emotionally. I regret not being able to be a support for her, although sometimes it was a bit overwhelming. I wonder if she will have time for reflection whilst LB is away.