First of all, YOU are a dear friend ... someone who reaches out for help for your friend is so admirable...
Let me just say that in my own experience of losing two children, no one knows quite what to do for someone they see in so much grief ,so unfortunately, most do or say nothing.. for fear of upsetting them. But believe me, it was the people in my life that talked to me, listened to me, that helped me so much, although I know it was not easy for them.
My children were younger when we lost our son, 5yrs old and 2 1/2yrs old.. but having raised them to now, teenagers, maybe I can offer some help anyway. I'm wondering if this 13yr old child is not feeling included in her parents grieving.. I'm wondering if maybe they are unintentionally excluding her from grieving together as a family. She may feel isolated and alone, not sure of how she is supposed to act or feel at all herself.. and maybe her behaviors are a way to reach out for someone to help her cope. It also helps to know that all people grieve in different ways..there is no wrong or right way to grieve. My daughter did not cry for a long time...which we worried about.. but in her own time, it finally poured out one day for no particular reason. We also went to counseling, however, our children spent time alone with the counselor which was of tremendous help to them... so it's important to find a counselor who specializes in children/families coping with death of a sibling. In my personal opinion, if they are no further in the grieving process after weekly sessions after a year, it may be time to find a new counselor. Please suggest to her the Compassionate Friends website... it is so helpful to know you are not alone in surviving the loss of a child...and there is also a special chat room for siblings as well. It is a wonderful resource which you might want to explore too, in learning how you can be of support to this family. Having good supportive friends is so helpful, but the flip side of that is that unfortunately we know they cannot really know what we are experiencing, which is no fault of anyone's. But knowing that, be there anyway as you have been...
God Bless... I extend my prayers and thoughts to all of you..