William is not an only child in these fics, but an older brother.

Admit One series by Cathey Scully
After The Truth series by Pattie
And Baby Makes Four by gillian dana
And Baby Makes Four Returns For Halloween! by gillian dana

Before Trick or Treat by 6friends6
Beyond The Truth: IV. Reopened by Neoxphile
Beyond The Truth: 4.25 The Witching Hour by Neoxphile
Beyond The Truth: 4.5 Thanksgiving Confessions by Neoxphile
Birthday Week by Neoxphile
The Burpday series by Kelly J. Thompson

Christmas in (Haven) Maine by Neoxphile and Faerax
A Christmas Wish by Erin Blair
Coming Home by YappiChick
Crystalic Millennia by Kyouryoku Senshi *Wip*

A Day in The Life - the complete series by Girlie_girl7
December 23rd by AnniePants
Destination: Truth by Kyouryoku Senshi & Billie Reid
Doom Child: Nature vs Nurture by Neoxphile adopted sibling
Doom Child: When She Was Bad by Neoxphile & Raesharra adopted sibling
Draw Down the Moon by Jenna Tooms half sibling

18 years and 1 day by Starbuckmsr
the Everything series by Eral C.

The Family G-Man by Neoxphile and FelineFemme
Family Files: V. Caitlin by Wildafox
Fate's Will by ElleThom
Father's Day by Ursula m/m slash
Father's Day II by Ursula m/m slash
Flight into Egypt by Vickie Moseley
Folie by Ursula m/m slash
For the Good of the Child by Aislinn Carter

Goodwill Towards Men by Neoxphile adopted siblings

Happy Birthday, Baby Boy by Squishy
Happy Birthday Mulder Triplets! by gillian dana
Happy 40th Scully! by Erin Blair

If Daddy Had Only Seen by Kelly J. Thompson
The Island of X by Tammy D. Aiken-Phillips

A Life by Jacquie LaVa
The Life They Now Lead by Lilxgirl(aka Abligail)

Miracle by Jennis524
Miracle Season by Neoxphile
Mist Over Water by Rae
Mulder House Rules by Sagan
Multiplicity by ElleThom
My Favorite Day by Mish

One For the Road by Starbuck_Jayne
The Only Broken Universe: Only Because He Needed Us, Broken Mosaic & The Dare by Neoxphile half siblings

Peace of Mind by Spooky2u2
Picking Up The Pieces by Gillyfan

Recovering Gemini by Neoxphile (being 4 days older counts, right?)
Resistance by Vickie Moseley

Sfumato by Supernova
A Sleepy Little Town series: Memory House by Neoxphile WIP
Sneakers by Gillian Leigh
Snow Globes and Seaships by Pattie
A Special Gift by ML
Surprise Packages by Jacquie LaVa
Sweet Dreams Series by Alice J. Foster

The Truth Is Not Enough Part 3: William by dana-maru1
Two of a Kind by Girlie_girl7

Unexpected by Twilight Walters

Water Water Everywhere by afg
We Who Remain Carry On by Neoxphile half sibling
Welcome to the Family by lil gillian
When Fate Takes A Hand by Madam F
William's Fifth Birthday by Erin Blair
Winterlong series by Neoxphile adopted siblings
Without You by Jennis524

Xavier by Becka F. adopted sibling

Zephyrs by Becka F. adopted sibling



There's a Nursery Files challenge on this topic too, so if you'd like to write a fic about William
getting a baby brother, go here for challenge details.

The Nursery Files