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.
I don't think it ever openly tells us that the parents are actually dead, it states on the TV that some are missing while showing Yuri's name, it's possible the funeral was held without bodies and Yuri is still alive.
From what we see no Christmas period passes and most of the show is between summer break and towards the end of autumn, we also see none of the girls birthdays promoting the idea that the show happens withing a two to three month period.
It is entirely possible that Yuri could still be alive as there is no solid evidence stating that they died, the news also stated that the plane crashed and was missing, again, if missing there is no guarantee that bodies were found.
For all we know, she could still be alive.
Also, Hina states that 'since mommy is in heaven, I'm bringing my drawing to show her' this implies that somewhere between episode 12 and 13 she came to terms and learned about it, also, another thing left out of the show is that Hina sees her parents either at her parents day or the musical show, in spirit anyway, though this isn't solid proof they are dead and could be just the imagination of a three year old it may have forced her to accept the fact her parents are gone.
