I lost a foster brother (not that he was anything but a brother as far as I was concerned) when I was 7-ish and he was 2. He had been ill and likely not to live long anyway (which I didn't know at the time - we knew he was profoundly disabled, but not that he was likely to die).
He died whilst my sister and brother and I were on holday with our dad (our parents were divorced) and he was with my mother and stepfather. By the time we came home from holiday the funeral was done and dusted, our schools had been told, his stuff had all gone etc. It was pretty devastating. He just disappeared from our lives. I now know that my mother and stepfather weren't given much choice about funeral arrangements etc, as he wasn't yet legally theirs - I can only imagine how harrowing the whole process must have been for them. They were basically invited to their own child's funeral, as it felt to them.
I do feel a bit resentful that they decided to take on a child with that level of disability and that prognosis, when they already had three children who had been through a divorce, several house/school moves, their father moving to the Middle East, and who were quite obviously suffering from a range of emotional problems (nervous cough, bedwetting, nightmares, poor behaviour at school, that sort of thing). Particularly as they went on very quickly to adopt another child with entirely different severe special needs which placed huge stress on the whole family.
I did feel very much at the time as though any grief I might feel was pushed aside, and that expressing it would be regarded as a kind of vulgar unforgiveable attention-seeking in the face of my mother's "real" loss and grief. Whether she intended it or not, that's how it felt. I've only recently started revisiting all that myself - I had nightmares about his for years afterwards and even concocted a really bizarre explanation for why it was my fault he died.
Sorry, I've rambled. I hope it helps for others to share their experiences.