Look at the picture. say who bill (mag) is example: bill is a son for his mother. mag is a daughter for her mother. his / her grandmother. his / her father. for his / her sister. 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.