I'd find
Theresa,
Sophia and
Emily more convincing; or
Tessa,
Sophie and
Emily.
Sophie seems to need a longer name when teamed up with
Theresa and
Emily.
I'm used to
Theresa; but since some Americans (most?) give
Anthony a -th- sound as in
Dorothy, perhaps
Teresa is safer?
Lara seems a bit flimsy; I'd use the more classic
Laura.
For a step-sister, I could see
Natalie, or perhaps even
Ashley.