@yurirei—
I didn't say that it were an impossible arrangement in the real world; I said that it would make for poor story-telling.
If she stays with her father and the other two, and they don't return to the first island, then either two stories have to be run in parallel, or one set of characters has to be set aside (much as her father and his two companions have been set aside). And, either way,
the set
of four characters for whom the story originally established the attachment of the readers is broken.
The reason that her father and his two companions have been off-screen is because telling
n stories of island survival (where
n>1) is just
cluttered unless something peculiarly interesting would be illustrated across the stories or the stories are going to be woven back together in some unexpected way. As it is, we may get occasional episodes of those other three, and they may indeed come to play some surprising rôles, but that's all that we should expect until and unless they move to be combined with the original four characters. And, in the case of parallel story-telling, having two survival experts supporting four characters (counting those two experts amongst the four) while having three non-experts stumbling on the other island would be imbalanced.
Setting aside the island with the two experts would set aside one the four characters to whom the readers are most attached. Setting aside the other island would do this to three of those four characters. What has so far been set aside are three characters to whom there is less attachment of the readers.