The correct answer is: between.
The preposition “between” is used to indicate that something is shared or divided among two or more people or things. In this case, the chocolates are being divided among the two sisters.
The preposition “among” is also used to indicate that something is shared or divided among two or more people or things. However, it is typically used when there are more than two people or things involved. In this case, there are only two sisters, so “between” is the more appropriate preposition.
The preposition “to” is used to indicate that something is given or directed to someone or something. In this case, the chocolates are not being given to the two sisters, they are being divided among them.
The preposition “for” is used to indicate that something is done or provided for someone or something. In this case, the chocolates are not being done or provided for the two sisters, they are being divided among them.