No, not directly after, not even definitely after, it could be before...you have to consider it's display order..
if the node you are creating a sibling of has a display order of 9 and you create a sibling and assign it a display order of 1 it will appear before it's sibling because 1 < 9 and it displays the lowest display order first, but both will appear within their parent.
Also before you get confused...in this diagram above...
-- A1
---- B1
---- B2
------ C1
------ C2
---- B3
The letters correspond to generations sort of in a way, A is a parent of all B , B is parent to all C, C is parent to all D and so on....
(Root)
-- A1
---- B1
---- B2
------ C1
------ D1
------ E1
------ F1
And so on...
---- B3
C is a child of B and D should be a child of C but the way you are displaying it you are showing D as a sibling to C which is incorrect.
Think of A as parents, B as children, C as grandchildren and D as great-grandchildren and so on
If you create a sibling of C1 you end up with C2 , if you create a child of C1 you end up with D1
(Root)
-- A1
---- B1
---- B2
------ C1
------ C2 ( = sibling of C1 )
-------- D1 ( = child of C1 )
---------- E1 ( = child of D1)
---------- E2 ( = sibling of E1 and also child of D1)
------------ F1
---- B3
what I was saying is if you have a node with a lot of children...that is when having the two different options can come in handy