Resposta:
1) Bella is not my sister.
2) He's not your father.
3) She is not my mother.
4) That flower isn't red.
5) I am not your brother.
6) They aren't in my house.
7) Kevin isn't her son.
8) It's not sunny today.
Is not, isn't e 's not são todos a mesma coisa, apenas abreviados de formas diferentes. Qual deles utilizar é totalmente opcional. Espero ter ajudado.
