I don't know how reliable it is either. I'm your daughter's size, and have a similar inseam. I have 3 bikes - a men's medium frame, a women's tall frame, and a road bike that's 54cm. I've been told I have very long femurs, lol. All of my bikes are quite comfortable. This calculator is telling me a small as a first choice. ?

If their bikes are unisex in sizing and the frames run large, then maybe. But you'd be hard pressed to convince me to buy a small...