No, I don't think they are too close for siblings.
Hayden and
Hayley would be too close, or
Hayden and
Hadley. But
Hayden and harper are ok. The only thing is, would you feel like you had to use a 3rd H- name for a 3rd child? If not, then go for it, but I personally would hate the idea of being trapped into a letter. FWIW, I think it'd be just fine if you had
Hayden,
Harper and then say...Teagan, or soemthing else, that was just off the top of my head.
Yay for
Hayden on a boy!
Hayden Edward is a very handsome combo.
Harper Jolee is nms.