She knows she has anorexia and that she's been very ill, has been hospitalised etc. we've never had really in depth conversations with her but as it's been going on for so long she's obviously been aware that things weren't normal for a long time (previously they were very close). My niece has just been diagnosed with ASD which means she has additional needs (and probably why traditional FBT treatments didn't work for her).
We don't see them often at all hand holiday was the first time in about 5 months.
As far as we tried to shield her, on holiday the restrictive and controlled eating was obvious, as was the codependent relationship with my SIL (my DD commented that they seem to be closer/she gets a lot more attention than with me and my DS).
After the holiday she showed me a photo from behind her and niece jumping into the water in their wetsuits and got upset saying how much fatter she looked.
Obviously have no idea if we handled the whole situation with my niece correctly, but she was only 9 when it started so we've gradually been a bit more open the older she has got.
We won't be getting the two of them together for the foreseeable future as I assume detrimental to both of them now.