Editorial Note: This article is written with editorial review and topic relevance in mind.
According to grammar rules, it should be this is she, because is is a linking verb (a verb that connects the subject to more information about the subject), so it can't have an object (her), but it's. The difference is that she's and similar shortened forms are used in colloquial speech, but not in certain cases. In short, she/they is the most common way for a person to indicate that they go by she/her or they/them pronouns, likely with a preference for the former.
Zamn She's 12? Know Your Meme
Taken from the free online dictionary: In your example, she is being emphasised.
