Below is a selection of British siblings with Willoughby, from the last century to modern times. The year relates to the decade of birth for that set.
As a surname-firstname, Willoughby has generally been used for boys in Britain. However it has, in past centuries, also been used for a few girls in Norfolk (including one of my great x4 grandmothers). All the Willoughbys below are male unless indicated with (g), where the Willoughby in the set is female.
Jessie (g) - dad was Jesse
Laura Rosalind - sometimes "Rosalind Laura"
Harriet E J
Joseph Esme & Willoughby James (twins)
2000s:Willoughby Robert John
Honor Matilda Alice
Beatrice Isabella May "Bea"
Willoughby Patrick Ernest
Orlando Alexander Rainsford
Willoughby Frederick Charles
Barnaby George & Amelie Tabitha (twins)
Willoughby Jasper Geoffrey