I don't love
Harper for either gender, but it bothers me less on a girl than most unisex names do, for some odd reason. I don't think it sounds good as a sister to
Piper at all--HarPER and
PiPER are way too similar, IMO.
Liberty &
Piper or
Liberty &
Harper would be fine, though.
BTW, I have a friend with a 7-month-old daughter named
Harper Annemarie.