Hey luffe, sounds a bit like me and my sil, she to has super ticklish feet which i have had the pleasure of tickling several times. She hates having her feet touched but never gets really mad after i do it and is such a good sport. It's a few years since I last tickled her feet but have been looking to find a chance to get her again recently.
Good luck with your tickling, I would love to hear about any tickling you done to her and any more in the future. If i get a chance of tickling my sil's feet i will let you know all about it.
Cheers M8