I am so sorry for your loss, Sarah18. I know every loss is different, but I feel something similar. My grandaughter died in January 2019. She was 9 years old and had a brain tumour. I am at the very start of a journey that seems to involve huge sadness, anger, disbelief, despair. How to navigate my relationship with my son who adored his daughter and I don't seem to be able to help. How to carry on loving and investing myself in the other grandchildren when I am so afraid.
As far as support goes, it seems to be mostly (understandably) focused on the parents, however I believe the Compassionate Friends talk to all family members when a child has died. I may try to contact them myself when I can get myself together.