Larkin is a guilty pleasure of mine. I like it on either a boy or girl. The girl names you have would all work well with it, but Ashby and
Brooks push it toward trendy and steal some of the quaint medieval charm.
Larkin Joel sounds like a musician or performer. I usually don't go for
Joel, but this pairing is rather cool.
Larkin Henry is a nice, solid combo.
For a boy:
Larkin JamesLarkin PatrickLarkin ConnorLarkin Fergus -
Larkin feels very Irish on a boy.
For a girl, I think the names you mentioned for a sister could also make nice middle names.