Is she still considered my sister?

No. She was never a she, never a person. You mom was likely to be able to tell that it was a female but, unless it was born, it was never a person. If was born at term or near to term and stillborn (born dead), then it would be considered a person, although born dead. In the case of a miscarriage, it was something that just did not make it to an age that would even have been considered as a person.

Yes, she is still your biological sister.

Your mother needs to acknowledge this fact as part of her health history. Whether you decide to is entirely up to you.

Legally No as she was not born, so you cannot put her on any documents.