Katherine, a great classic name but I prefer the spelling Katharine as apparently it's closest to the Greek original form of the name. Whereas Catherine can be mixed up with other names like Caroline and Christine. I speak from experience.
Alexandra is also a lovely name. Not so sure about all the short forms though as Alex is a bit unisex and I have heard Lexi described as 'chavvy', Sandra as dated, ditto Sandy (and also a bit gender neutral) so what would you use?