The designator prevSibling describes the sibling note in $OutlineOrder before the note in current focus; the previous note on the same outline level with the same parent, i.e. excluding children.
Note that whilst prevSibling is the normal assumed spelling, the alternate of previousSibling is supported, though deprecated.
Designators don't allow offset references as with attributes. Thus $Name("Some note") works but child("Some note") doesn't. If an offset designator is needed use find() instead.