Neither of them would be my choice. I far prefer
Louise to
Louisa, but even then, not enough to use. And I prefer just about any
Lin- or
Lyn- name to flimsy
Linnea.
I don't think they're too similar, though. Similar enough to show that the same parent chose them both, but not cloyingly matchy.
If I had to, I'd seriously consider
Eloise, Lyndall and
Lucy.