What the characters of the fours sisters were = OBJECT of the verb structure "we will leave to to be found out" You could rewrite the sentence: We will leave [what the characters of the four sisters were] (object) to be found out. As you can see, that sentence structure is a bit cumbersome. The authour chose a specific style, one that raises a question and answers it in one stroke. Think about it.

What the characters of the four sisters were -->how the sisters would behave or what values they would exhibit we will leave to be found out-->you will discover as you get on with the movie Let's wait for the teachers to explain further. :)

The subject is not 'What the characters of the four sisters were.' The subject is 'we'. Take a look at the verb 'will leave'. Who will leave?' 'we' will leave! So 'we' is the subject. What will we leave to be found out? What the character of the four girls were. So this is the object! This is a noun clause starting with a question word. Noun clauses cannot be in question form. They must be statements so we use the same structure as indirect questions.

I think it could be: What the characters of the four sisters were, we will leave that to be found out. It means that the author wants you to keep reading the novel in order to discover the characters of the 4 sisters and what they were like.