Hey man,
Just tickle her feet when the opportunity looks right and play it off like she was asking for it by having her feet all exposed. Don't ask her if you can tickle her. If she responds reasonably to the first tickle (no cursing or bitching), just tickle her at a later date. Make it look spontaneous. If you ask her if you can tickle, it looks like you are into tickling or feet or both, and that would probably freak her out.
As an aside, my college roomate's sister had really ticklish (and cute) feet. I never asked her if I could tickle her. Instead, I would theaten her with tickling when she would be obnoxious. I would say, "Denise, if you don't quit, I am gonna tickle you". Then, she would beg not to be tickled, and it would turn into sort of a game, which ended up in her being tickled. I have a feeling she knew I liked tickling feet, but since I did not couch it those terms, it never seemed weird to her when I did tickle her feet. There was always some justification (her misbehavior).
If your step sister doesn't misbehave, just make something up. Tell her you don't like the channel she is watching on TV. Tell her you don't like her putting her feet up in the couch, cause that's where you lay your head. Doesn't matter what it is. Just come up with something other than, "May I tickle your feet". A statement like that immediately throws up flags of sexual gratification.
Good luck hunting,
Confessor
