Because it is not important in this case in french language about who you are talking - about male or female..doesn't matter. That's why you have to say like it is male, like usual

"C'" means "this thing" or "this person" So, "C'est ma soeur" means "This person is my sister" "C'est ma maison" means "this thing is my house" "Cet" or "Cette" only means "This":

First, "cett" does not exist in french, that's not a word, no sense. C'est = It is, This is, That is So you can say : Cette fille est ma soeur or : C'est ma sœur . in english you use "this is", in french "c'est" Translation word to word in not always the good way, it's better to think in the language, and learn common expressions and ways to make sentences correctly in the language you learn.