No, Trump’s sister did not publicly back him. He was duped by a fake account.

Source: https://www.nytimes.com/2020/11/20/technology/no-trumps-sister-did-not-publicly-back-him-he-was-duped-by-a-fake-account.html
November 21, 2020 at 12:54AM

The president shared an article that said his sister, Elizabeth Trump Grau, was supporting his claim that he had won the election. But the article was based on posts from an impostor Twitter account.