Depending on what property you have available, you have a few properties that you could use.
If you pass a step object into your edit substep, you can use Step.Sequence.SequenceFile.Path
If you pass a sequence context into your edit substep, you can use SequenceContext.SequenceFile.Path
Be careful using this property, as it is common to add steps that belong to a sequence file that has not been saved. In this case, the Path property will return an empty string.
Hope this helps!
Allen P.
NI