Oh gosh, I really do feel for you springy. I have to say the only reason I thought that is my second DD was born sleeping at 37 weeks 5 months ago.
I would appreciate your contribution, I honestly would. I imagine though, that if she is bitter, that she will lash out, and unfortunately you are in the firing line. I wouldn't dwell on whether she appreciates it or not, I'm sure she does, but the other comments, have diminished her loss, and that is why she is so dictatorial over it. To be honest she will never get over it. Even those who appear to, only wear a mask, they may appear fine to others, but no one will ever realise the depth of their grief. It doesn't get better.
Did your sister ever have counselling? It really sounds as if she could do with some, even now - it may sound silly, but she has obviously got trouble in coming to terms with it - in fact I don't think she has.
She's probably a scared, woman, in pain. Who misses her little girl, the world has moved on, she's been given the impression (by the insensitive comments) that how she feels is wrong, and wants - desperately - to keep her baby's memory alive... which anyone who's lost a child knows. Most of us come to realise that we need to wear a mask, and pretend to be fine, your sister still has her grief very close to the surface.
Have you ever contacted SANDS? They could perhaps help you, as well as your sister. I don't know how you'd broach the subject with her though.
If you send a card, maybe print off the poem 'Little Snowdrop'. I have no idea if your sister will appreciate it, but it's a lovely poem, and will show that you care, and that your niece meant something to you x