I have no definite answer. It could be based on a true story, but it could also be purely the creation of the author's imagination. You might want to check reliable sources or reviews to get a clearer picture.
Well, it could be or it might not be. To know for sure, you'd need to look into the background and the author's statements about it. Sometimes inspiration comes from real life, but the story gets embellished for entertainment.
Family is a big theme that could be in it. The relationships within the family are likely to be strained and changed by the mother's disappearance. Siblings might grow closer or further apart, and the father's role might change dramatically. Also, there could be a theme of resilience, as the family tries to carry on and find answers in the face of this difficult situation. It may show how they cope with the unknown and keep going in their daily lives.
Partly. The movie takes elements from various real cases and weaves them into a fictional narrative. So, it's not a direct retelling of one specific true story, but has roots in reality.