I see this as Mary J describes it. While I think some of those feelings are normal as a young person is going through puberty and especially moving into young womanhood-- women get mixed messages about being "shadows" versus asserting their own will--I think for both Phoebe and Marithe this was exacerbated by their fragmented family situation. In one way or another, their father is absent, not there to affirm and support their emerging selves. It's important for either gender to be affirmed by both mother and father, or mother figure and father figure. Just having a strong mother (reclusive or not) isn't enough. Niall and Ari seem to have filled this role for their sisters to some degree. I know when my daughters lost their father, my stepsons did the same for them. All of the children in this book seemed to be good, loving and distinctive personalities, and I was glad when Daniel thanked Claudette at the end for doing a great job of holding them together.
It's interesting that we don't hear as much from Phoebe or Marithe as from Daniel's sons. So it's hard to know how much they might have in common besides physical appearance, and a largely absent father. There are hints that they might share their father's sensitivity to nuances and his inward reserve. Marithe seems a bit more outgoing than Phoebe, a bit freer and not as likely to go with a crowd she doesn't like, but of course she hasn't been put into any situation where that would be an option.
Niall and Ari both seem to have much in common: very smart, very comfortable with and insistent upon being their own unique selves, confident in ways Daniel is not, and yet like their father, very loving and good with children. Each of them, of course, has had to learn to live with a challenging, chronic physical problem.