Answer

The answer is 5. As explained in the hint, you can solve this puzzle with just two sentences. However, the other information is complimentary and this can be proven through algebra.

Method

To solve the puzzle, first examine each sentence and substitute into separate equations the information. Use a for the protagonist, s for the older sister, and m and d for the respective parents.

The first two sentences explain the scenario and is of little use, apart from providing which people are involved. The third sentence can be formed into an equation, though. It tells us that minusing her dad's age from her mothers (or the difference between them) is her age. This is represented mathematically as a = d - m. The fourth sentence explains that her sister is twice as older than her, or s = 2a, a = s/2, and that her older sister is one-third of her mother's age; s = 1/3m. The last sentence is vital, as it explains that in five years time, the child will be her sister's age: s = a + 5.

With this information, to solve the conundrum, all you need to do is rearrange the equations given, specifically the equations deduced from the fourth and fifth sentence. Replacing s in the second equation with 1/2a is all you need to do, and it becomes clear that half-of the sister's age is 5, which is the same age as the protagonist.

Like most of these, it's relatively straightforward once you remove the excess, unneeded information which make puzzles like this much harder than they seem.