The one with more buff and white on her back is Ivy:
Her face hasn't changed much, although she's grown a little bigger, since she was the year-old kitten shown here:
(Why won't the system line these pictures up in a reasonable way? I have no idea.)
Irene, sitting behind Ivy and beside the mowing blade above, displays her back here:
And here's Irene's face. Note that Irene's face is not symmetrical and does not have conspicuous eyebrow and eyelash markings.