When does Hina learn that her parents died? The parents of the Takanashi siblings die in a plane crash. Sora and Miu are told about this immediately, but as far as I remember in the anime, Hina was never told this. When she asks when her parents are coming back in one of the earlier episodes, the other characters don't explain this to her directly, and she doesn't understand the meaning of the funeral that they go to.

Is it ever properly explained to her why her parents aren't coming back (perhaps in one of the later light novels)?
At episode 12, since her performance at the Daycare approached, Hina began asking again when her parents will be coming, prompting the others to reveal the truth.