Do I need to remove peanut butter/oil stain from unfinished wood before staining?
You say a dark stain, which is good, but the peanut oil has already filled some of the space where the stain can soak in. That's bad. You don't want visibly lighter handprints on your steps.
I'd go with rags liberally moistened with mineral spirits. Let them sit on the spots for half an hour or so, then scrub vigorously.
You'll know if you succeeded when the mineral spirits dry off over the course of several hours.

It's best to have some ventilation when dealing with mineral spirits.