Vertical menu or module with neighbor categories on the same level

[gajdi]

I would like to create a module to display the neighbor categories on the same level depth.

-Main
--1
--2
---a
---b
---c
--3
---A
---B
---C

So when somebody visits 2/a, the module should show b and c.
And when 3/A is visited show B and C.

Basically is somebody tells me how to get the actual category's parent's ID, that would be great too.

If any existing modules/menus/plugins exist, I am interested in them. (Even commercial)

I use J2.5.9 and VM 2.0.20b.

Thank in advance.