They sound like a pair of dogs rather than sisters.
My name is
Millie, so for that to come from someone bearing one of these names I think it's bad. I love my name. I love
Millie to pieces. But paired with
Poppy I can't get the association with dogs out of my head. They sound awfully cutesy and young together.
Penelope and
Millicent "Poppy and
Millie" would probably be better. Or
Millie and something else, something stronger such as
Ruth,
Beth,
Nina,
Megan etc :)