Marcos,
I do not think there is anything in the DWH to identify the wbs sequence.
In the prtask table fields such as
PRWBSLEVEL - tells which level
WBS_NNBSEQ - tells the next branch in the sequence
WBS_PARSEQ - parent task
You can check the PRTASK table it does explain these.
You also can do an action trace on a small wbs with children and see the query it returns.
PRWBSSEQUENCE - sequence in the level