For those of you who were kind enough to help and comment on my traumatic situation regarding having to choose between attending my DSis wedding vs supporting my son who had been treated appallingly by her, I thought I'd let you know what happened in the end.
So I went downunder and visited my son. I chose to support my DS, but let my DSis know that I would always be there for her in the future. I tried to arrange to meet her and discuss face to face but she would not see me, and sent me texts and emails telling me "you and your family are dead to me" and also "if you turn up at my wedding I will call security" (!!). It was extremely upsetting.
Then DM arrived for the event, and she sent numerous upsetting emails telling me the stress of what I was doing was causing her to be ill, and that I should just get over what DSis had done and all have a happy time together. She also broke several confidences which I had told her, regarding my view of what DSis and her DP had done (which made matters unnecessarily worse), then denied that she had done this, then when I spelt it out to her she said that she didn't know they were confidences (it was obvious). So I felt very betrayed and that she was favouring my sister (who can never do wrong, anyway) over me and her grandson. Finally, when I wouldn't give in to what she thought I would do, she wrote to my brother and told him she was worried about my mental health. The whole thing was really horrible.
Anyway, the wedding was on Saturday. I haven't heard anything from anyone except that today mother sent me an email with nothing written in it, just photo after photo. Why did she do that? Why not write something, e.g. we had a lovely day, thought you would like to see the pics; or sorry you weren't there; or just something. Why send these photos accompanied by radio silence? I don't get it.
So, I'm off home tomorrow, leaving behind me a completely shattered family. No sister, no mother. All extremely upsetting, but I do feel very confident that I did the right thing, so thanks to everyone for their advice. If anyone can throw light on the general behaviour I've been living through, which has been a complete nightmare, I'd appreciate it.