Personally I wouldn't want more than two siblings to have the same initial (unless I had like twelve kids, then three or four wouldn't be such a big deal, but that'd never happen!). But I'd use V or F:
V: Victoria, Virginia, Viola, Vidalia, Vienna, Venezia, Valencia/Valentina or Valentino/Valentine, Viridian, Valerian (wow, lots of city names)
F: Felicity, Francesca, Fidelia, Fitzwilliam, Ferdinand, Faramond
Hmm, I think I'd have an easier time coming up with one favourite from each letter of the alphabet...
Upon mulling it over, I realize that while I favor J and M names, I also seem very partial to names that begin with a vowel and end with an 's' sound. Atlas, Iris, Alice, and Eloise are firm favorites. I also like Ellis, Augustus, Emrys, Amadeus, Artemis, and Agnes. Felix, Hollis, and Wallace begin with consonants but end with the same 's'. This partiality is a problem since my last name begins with a 'Z'! Does anyone else find themselves drawn to a sound pattern like this?
I would probably choose A or H
Allifair, Apolline, Adair, Autumn, Adelaide, Arietta, Araminta, Alice, Amoret, Annabeth/ Alexander, August, Archer, Asher, Amos, Arthur, Abram, Alfred, Alden, Atticus
Hannah, Hazel, Holiday, Helen, Henrietta, Harriet, Haven, Holland, Hermione, Honor, Hope/ Haddon, Hayden, Hale, Hallam, Hart, Harvey, Hayes, Heath, Henry, Henley, Holden, Hugo, Holt