Bonjour,

Réponse :

1. Is your sister watching the play ?

  Yes, she is.

2. Are your friends going to the circus ?

   No, they aren't.

3. Are they making a concert ?

   Yes, they are.

4. Is her cousin playing in the circus ?

   No, he isn't.

Explications :

Question:

              Is / Are + sujet + V-ing ...?

Réponse courte :

              Yes, pronom + is /are

               No, pronom + isn't /aren't.