There are many relationships in Miss Peregrine's Home for Peculiar Children, Hollow City and Library of Souls by Ransom Riggs.
I have selected all the Peculiar Children and added their relationships with others:
Emma Bloom - Abe Portman, Jacob Portman
Jacob Portman - Emma Bloom
Miss Peregrine - All her peculiar children
Hugh - Fiona
Fiona - Hugh
Millard - He's lonely . . . (tear tumbles down cheek
Horace - He's lonely . . . (tear tumbles down cheek)
Of course, the twins share a special bond. We don't even need to mention that. It's too obvious.