I've only found one picture that is supposed to be her in her new role as villainess 'Alexandra' (as far as I know, she must be a new character in the Defenders/Marvel pantheon.)
From Marvel PR back in October:
Golden Globe-winning and Oscar and Emmy nominated actress Sigourney Weaver was announced as playing a significant but unspecified role in “Marvel’s The Defenders.”
“Marvel is honored to have an actress of her caliber and status join what is already our biggest show ever! She is by definition a class act,” said Jeph Loeb, Executive Producer and Head of Marvel Television.
“Sigourney Weaver, what more is there to say,” said Cindy Holland,Vice President of original content at Netflix. “She’s a formidable actress and we are thrilled to have her join our remarkable cast alongside Charlie, Krysten, Mike and Finn.”