Tabitha and Reuben's sibling is called...

[SmeleanorSmellstrop]

... what?!!

Oscar
Nicholas
Oliver

Gorgeous names.
I'm not sure whether the biblical theme is accidental or deliberate so I've gone for 2/3 of each.
Tabitha, Reuben and...

Martha
Lydia
Constance/Connie
Miranda
Cassia

Ezra
Gabriel
Hugo
Lucien
Caspian

Just seen your update.
My mum used to know someone whose children were Tabitha, Jemima and Joseph.
I think Tabitha and Jemima are great together.

Percy

Penelope

Lovely names OP!

It has to end in a different sound.

So ...

Tabitha, Reuben and:

Jeremy
or
Kate

Tabitha Reuben and Jake
Tabitha Rueben and Phaedra

Mallory
Emery
Vanessa
Samantha
Matilde

Jesse
Vincent
Quintin
Seth
Ezra
Simon
Jude

Arthur
Jude
Isaac
Noah
Dexter
Charlie

Grace
Eleanor
Emily
Imogen
Charlotte
Matilda

Tallulah or Piers

Roy

So many ideas! Thank you everyone - loads to add to the list!

Margot

Hi OP, I was the third Bob, no Mumsnet code that I am aware of! It just really tickled me when someone suggested a less ahem frilly name for sibling. I do like the name too. Not very helpful I guess, sorry about that.

Hugo and Ottilie

Lovely names!

Malachi is pronounced Mala Kai. I Love this name.
Gabriel, Tobias, Caleb, Raphael,

Jemimah, Eden, Shiloh, Naomi.

Girl Scarlett.

Boy audie.

Tabben

Naomi
Isabella
Virginia
Elena
Phoebe
Nerissa
Delilah
Elizabeth
Scarlet
Victoria
Beatrice
Caitlin
Bethany
Madeleine
Eunice
Arielle
Alexis
Cecelia.
Nadine
Felicity
Joanna
Anastasia

Alexander.
Francis
Charles
Joshua
Tobias
Daniel
Nathan
Elijah
Elliot
Jacob
Moses
Gabriel
Malachi
Theodore
William
Lucas
Jackson
Isaac
Ethan
Leo
Dominic
Christopher
Finbar
Finley

I know siblings called Tabitha and Reuben! Their
other siblings are called Jasper and Annabelle.

Esther
Robyn
Frances

Fay