The boy, ... is playing football, is my brother. (which or who) The teacher ... the boy to stand up. (said or told)

The boy, who is playing football, is my brother. - Мальчик, который играет в футбол, - мой брат.

The teacher told the boy to stand up. - Учитель сказал мальчику встать.

Who is used with persons, which is used with objects or animals.

The second sentence is imperative construction, indirect speech. We don't use said in such sentence.