J.L.
Haha! I've been dealing with the same thing. My lil one wants to wear socks & shoes all the time. I can't stand the sight of socks with sandals. I wiggle my toes around, tell him how comfy it is to walk on the carpet with no shoes/socks, take my shoes off in the grass, tickle his feet... and he's finally come around, I think it just takes a little getting used to. Seeing you and her siblings in sandals is the best way to teach. They probably don't remember last summer and wearing sandals then.
I don't know much about the sensory disorder for the hypersensitive except for what I've read on a blog - one of the resident bloggers at babycenter.com has a daughter with this problem. From what I understand a lot of these kids actually feel pain when you put them in clothes made of certain materials.