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

The sentence should be: "This IS my sister"

Because:

I  -  positive "AM" or negative "am not or Im not"

He, She or it - positive "IS" ... or negative "is not or isnt"

We, you, they -  positive "ARE"... or negative "are not or arent".

Its the verb "to be" in the present.