my sister and annorexia

i've never really spoke about this on here or if at all, but this christmas things have really hit home with my sisters eating disorder, we've all known she's had eating problems for an extremly long time, but it's really hit home this christmas. She's lost more weight, and now mum and dad are arguing over it and are threatening to put my sister into a pyschiatric hospital. I'm not sure what to do how to deal with this at all.