Pearl's sister is called...

[ladybranstonpickle]

I am totally stuck! Any ideas?

Cora and Pearl sounds good to me. Similar feel without being too similar in sound or meaning (I think too jewel-names like Pearl with Ruby or Opal sounds a bit clunky). I don't think I'd like to be in quite such a "matching pair" with a sib like that meaning-wise if it was me being named.

Otherwise the Iris and Ivy suggestions are nice ones, or Violet.

Pearl and Josephine? Somehow work together without being similar in length or sound.

Evelyn

Ugh hate the suggestion of Ruby and Pearl - sounds like a children’s book!

Winnie
Scarlet
Phoebe
Daisy

I'd totally keep the gemstone theme so: Ruby, Opal or Emerald.
Or another nature one: Ivy or Willow, I like Fern but rhymes a bit.
Or wildcard : Prudence

I wouldn't choose a gemstone name, too twee and matchy.

I'd go for Pearl and
Margo
Sadie
Nancy
Clara
Naomi

Pearl and Lillian 🥰

Margaret (or versions of) means Pearl so might be a bit weird

Abigail

By Jacqueline Wilson.

Penny

Coral, sea connection, precious material connection, lovely name.

I know a Pearl whose sisters are Penelope and Phoebe

Oyster
Grit

Dreadful

Agree! Cora is lovely, and goes well.

The Pearl I know has a sister called Florence.

Beryl or Opal

Coral

Rose

Edith

Faith

Joy

I was at school with sisters called Pearl and Penny.

Another vote for Nancy

Ethel

Pearl and Sylvie
Pearl and Miriam
Pearl and Anthea
Pearl and Una
Pearl and Meredith
Pearl and Allegra
Pearl and Tessa

I'd avoid precious or semi-precious stone names, which would be far too matchy.

Nature names might go well:
Seren
Eden
Daphne
Sylvie
Iris
Flora
Laurel
Willow
Hazel
Cecily
Estelle