They're talking about her father.

The person the person the boy saw is the girl's mum.

You point to a person 15 metres away and say: …… 是 谁？

You are holding your family photo in your hand and say to your friend: …… 是 我 爸爸、…… 是 我 妈妈。

You introduce your sister to your friend: …… 是 我 妹妹。

You look down the hallway and think you recognise someone: …… 是 你 哥哥 吗？