Thanks all. I'm so sorry that this has been other people's experiences too; and it's such a mindfuck to have something which is ostensibly couched in terms of care and interest be anything but.
@binkie163 You're right - I think grey rock just isn't working when DM starts working on something like a dog with a bone, and bluntness is one route forward.
@AttilaTheMeerkat Re NC: yes, it's mainly for family. My DN is under 10 and after they were born, we started doing more stuff as a family together. I know sibling finds parents as difficult as I do, but also wants to show a complete family together for DN; sibling's partner is not from UK so they don't have support from in-laws around DN, babysitting etc. But I do need to have a word with sibling about it. I love sibling + their partner + DN dearly, but was much happier when I had less contact with parents.
@CheekySnake 'I don't want to talk about it' is excellent, thank-you. Like you, I don't trust her at all not to share my private info or use it as a weapon. I do see my sibling without parents, but as above, sibling wants to have complete family things too. The fact that she cannot understand or respect my right to privacy is actually really fucked up, now I think about it!
and yes! to being 'helpful'. You've just reminded me of a time when I had to be at my parents en route to see another friend in the same town. DF offered to drive me there, I said thank-you but no, is quicker to walk. DM got into a complete lather about times, safety, when we should leave, roads, etc etc etc that had nothing to do with me getting from point A to B and everything to do with her inserting herself into a situation that is nothing to do with her, and making it all about her.
@Twatalert (excellent name). It's never about actual interest, is it? DM professes a huge interest in DN and yet when sibling announced the pregancy (first grandchild, I have no children). DM listened for half a second then veered off into an anecdote about someone she'd known once and their baby and blah blah blah.