Вставь am. is. are. This ... my sister.

The sentence should be: "This IS my sister"

Because:

I - positive "AM" or negative "am not or I'm not"

He, She or it - positive "IS" ... or negative "is not or isn't"

We, you, they - positive "ARE" ... or negative "are not or aren't".

It's the verb "to be" in the present.