Sister for Louis...

[catinboots]

DC2 due in September and we are struggling with girls names!! Current favourite is Ava. Also like Agatha but WIDE open to suggestions. We have a one-syllable surname beginning with H....

[mathanxiety]

Love Agatha. Also Agnes. I love Beatrice too.

[kimbles1984]

I know a Louis with a sister called Felicity

[doughnutty]

As Louis has a french feel how about something of that nature.
Celine
Cecile
Elodie
Emilie
Genevieve
Juliette
Marianne (love this)

[recireci]

Evelyn
Sophia
Abigail
Alice
Jessica
Imogen
Caroline
Aurora
Clementine
Emmeline

[janeite]

Agatha nicer than Ava imho.

Cecily, Esme or Verity nice, mayhap?

[sparklyrainbow]

Rachel
Charlotte
Eleanor (Nell)

think you need to avoid an 'e' sound at the end or as a pair sounds a bit twee IMO

[sunandmoon]

My french nephew who is called Louis has a little sister called Camille.

[bellissima]

My Belgian nephew Louis has a sister Marie.

(Although - sparkly's comment - actually both Marie and Camille sound a bit 'ee-ish' at the end. I like Charlotte or Marianne or Harriet or Caroline.)

[mathanxiety]

Marie is lovely, imo, and it has a longer EE sound at the end than Louis does, to my ear anyhow, so not too matchy. Camille does too.

[GladioliBuckets]

Oh definitely go French!

Alyette
Claudine
Delphine
Solange
Charlotte
Anais
Heloise with a silent H
Belle
Severine
Éva instead of Ava
Adele
Madeline
Seraphine
Vivienne
Amandine
Chantal
Yvette