Przykładowa odpowiedź:

The problem in the letters is that two siblings can't get along in one room with each other.

The solution for the letter A could be for writer's sister to relax a little. She may feel a bit cramped but she should compromise a little. Writer could learn a thing or two from her sister too. If it's such an issue, she could try to by more tidy and put borrowed things on their place.

The solution for letter B could be for writer to step down. Schoolwork is very important and he should respect his brother's efforts. They could arrange a schedule, when would it be OK to play loud music and when it's time to focus for both of the brothers.