Analogue of "orphan" for someone who's lost their siblings
They are often mentioned as "bereaved sibling".
For example, in this book:
Sibling Grief: Healing After the Death of a Sister Or Brother - By P. Gill White
bereaved adj. Suffering the loss of a loved one: the bereaved family.
Someone with no siblings is called an only child. I have heard an elderly woman say, when her last sibling had died, "Now I have become an only child."