Some of you know my backstory but the important bit to understand is that through my DH and DSD being subjected to parental alienation by his ex/ her mother - we no longer see DSD who is 13.
I have been with her dad for 5 years now so my DD who is 6 doesn't remember a time when DSD wasn't around and has always happily accepted her as part of the family. They called each other sister and DSD was here for 50% of the time up until December when contact dropped to one day a week and since May has been zero.
DD was very upset at first but seemed to be coping well of late. But over the last few weeks her behaviour as deteriorated again. She told me a few days ago that she thinks about her "sister" all the time and feels like she is dead. She said she sometimes thinks about her at school and forgets what she is supposed to be doing then gets told off.
She said she sometimes feels angry at her step dad, even though she doesn't think it's his fault but most of the time is just angry at her "sister". And sometimes she prefers to be at her Dad's as beng here reminds her of how it used to be 
She is clearly really affected by it all and I don't really know what to do. I think I will take her to the doctor to see if they can refer us to some kind of counselling but not sure if that's right or not... any advice out there?