Is there a single word to describe the loss of a sibling?

No there isn't.

You wrote "Without a proper name it is hard for the grief to be acknowledged"

"loss of a sibling" should be more than enough for someone to acknowledge the grief, even to those without siblings themselves.

You have only asked for a word to describe the situation which means there is a chance that you yourself have lost a sibling, and that is enough for me to feel a deep sense of remorse. So as I said, simply stating that you have lost a sibling should be more than enough.

There is a word (which is to be used loosely as a transposition from the animal kingdom) for the killing of one's sibling: Siblicide.

More specifically Fratricide for the killing of a brother and Sororicide for the killing of a sister.

Unfortunately there isn't a word for the loss of a sibling although now that you have mentioned it, I feel as though it would be a good idea to have one.

If it is indeed the case that you have lost a sibling then I wish you all the best and I have no doubt everyone else reading your question shares the sentiment.