1.I love the name, seeing as how it is my stepfather's surname, and he's a great guy.
2.Definitely girl.
Harper Lee made it feminine, and it has a rural, quirky kind of charm.
3.Neil
Harper,
Desmond Harper,
Michael Harper Harper Michael,
Harper Quinn,
Harper James4.I think
Harper and
Atticus are a little TOO matchy-matchy for siblings. Its a good theme, but if you're going to go with either name, an equally quirky yet unrelated to the book name would be better.