The boy is taller than his little sister.

or, more grammatically,

The boy's little sister is shorter than he (is).

To determine who is taller between the boy and his little sister, we compare their heights. This sentence states that the boy is taller than his little sister. To find out who is taller, you could compare their heights directly by measuring them. Place each child against a wall and use a measuring tape or ruler to measure their heights from the ground to the top of their heads. By comparing these measurements, you will be able to determine who is taller.

The boys little sister is shorter than him?

Yes, I agree with the people above me.