Look at the picture. Say who Bill (Mag) is for...

Example: Bill is a son for his mother.
Mag is a daughter for her mother.

...for his / her grandmother.
...for his / her father.
... for his / her sister.
...for his / her grandfather.

Bill is a grandson for his grandmother/grandfather 
Bill is a son for his father
Bill is a brother for his sister
У Мэг всё наоборот: для отца она daughter, для бабушки/дедушки то же самое но с приставкой grand. Для сестры она, я думаю, сестра.