We are a family of four.
私の家族は4人家族です

This is Emi, Kai’s friend, this is first time to send email to you.
私はKaiの友達のEmiです。今回初めてメールをお送りします

I have elder brother and sister, and I’m last-born.

兄弟は上にお兄さんとお姉さんがいて、私は末っ子です

I have elder brother and sister, and I’m last-born.