A variant of Paige is Paget or Pagett, and some similar sounding names are Payce/Paise, Paz, or Peace.
Names with similar meanings ('servant/helper'): Abelarda, Abigail, Ezri, Gilda, Gillian, Nasira, Sia
Variant/short forms of the Kristina (not including Chris) are Tiana, Stine (STEEN), Tina(h), Kine, Ina, Kit, Karsta, Carsten
Names with similar meanings ('christian woman'): Trinity (associated with the Christian doctrine), Heaven, Christian virtues: Faith, Hope, Charity, Constance
As previous poster said, you could always use the same initials, or perhaps use a name that has to do with something your sister likes