—My pen is lost and I can’t find it anywhere. 一S0 you will have to buy      . 

Nora is an American girl. She studies in a middle school. She has a little sister. Her name is Kate. Kate is only four. Nora likes Kate very much.
Today is Sunday. Nora wants her pen. She takes out her pencil case. “Oh, dear! Where’s my pen?” She can’t find her pen. She goes to ask her sister in her bedroom.
“Kate! I can’t find my pen. Can you see……Oh, what are you doing with (用) my pen?”
“I’m writing to my friend, Betty.” Kate answers. “But how can you? You don’t know what to write.”.“It doesn’t matter (没关系). Betty can’t read.” Kate says.
【小题1】Who are sisters? _______________