Sibling is theoretical, so I'll take boy or girl names.
Frederick currently goes by both his full name and nn Freddie.
Preferences: classic/old-fashioned, names that don't end in a vowel, nothing that begins or ends in "L", nothing from the top 50, standard spelling and pronunciation
Re: Sib for Frederick?
My furchildren, Dorothy and Johnny
Roland
Clarence
Hector
Ignatius
Walter
Edgar
Girl names not ending in a vowel is a bit trickier
Agnes
Constance
Edith
Harriet
Maureen
Millicent
Rosalind
Sybil
Theodore, Franklin, Nicholas, Charles, Wesley, Elliot, Alexander, Peter, Patrick, Vincent, Martin, Calvin, Jasper, Tobias, Wilbur, Simon, Leland, Sylvester.
Wendy and Ramona might make me lift the ending-in-a-vowel rule.
Evangeline
Margaret
Josephine
Willa
Bridget
Frances/Francesca
Carmen
Justine/Janine
Gretchen (Greta)
Helen(a)
Kathleen
Dianne
Juliet
Clementine
Florence
Susanna
Garrett
Seth
Darius
Maximillian
Gerald
Reginald
Phineas
Peter
Winston
Emmett
Augustus
Albert
Kenneth
Harrison
Arnold
Stuart
Douglas
Julian
Adrian
Roger
Edwin
Terrence
Wilbert
Joseph
Bartholomew, Sebastian, George, Blaise, Ambrose, Ignatius, Phineas, Edmund, Desmond, Jude, Julian, Harlan, Jasper
Frederick and Marjorie
Sib is now slightly less theoretical (BFP on the HPT last week!), so I'll definitely be coming back to this.