Is it permissible to register one of the brother's children in the name of the employee so that he may benefit from his salary after his death, knowing that the employee's salary will be completely cut off upon his death and his wife, who is a teacher, cannot obtain any part of it?

What the questioner seeks is falsehood, deception, and the unlawful consumption of wealth, and it involves attributing a child to other than his father. All of this is impermissible in Islamic law, except in cases of extreme necessity, such as when there is a fear for the child's destruction or death by starvation. It does not appear that what the questioner mentioned reaches this extent, so the prohibition remains.