If you are holidaying, perhaps portability should be a factor. If so, consider the mirrorless brands like the Panasonic G2, GF1, G10 etc. or Olympus EPL-1 or EP2, Sony NEX or Samsung NX10.
If your sister has a significant investment in lenses then purchasing a Canon is probably the best way I guess. But IMO, a good 18-55mm range is versatile enough really. You don't want to be continuously changing lenses on the move.