学习笔记：
father：爸爸
例句：His father was a professor of medieval studies.
他的父亲是研究中世纪历史的教授。
mother：妈妈
例句：He recognized his mother's handwriting on the envelope.
他认出信封上他母亲的手记。
brother：哥哥
例句：Must your brother tag along with us everywhere we go?
你兄弟干吗总紧跟着我们?
sister：姐姐
例句：She was intriguing with her sister against her mother.
她和妹妹串通捣鬼和母亲作对。
baby：婴儿
例句：I must mind the baby while Mum goes to the shops.
妈妈去买东西时我必须照看这婴儿。