Honestly I don't think Marilyn was in a position where she could've reliably stood up for Vivian evem though she may have wanted to. Sure, Beldam treats Marilyn better on the surface, but she also has a special insidious level of control over Marilyn, since Marilyn is mostly nonverbal. Marilyn doesn't have any alternative forms of communication, and I'm almost positive that never providing alt forms of communication is intentional on Beldam's part. It means that Marilyn can't easily speak back against Beldam, and worse, if Beldam threatens to ditch Marilyn, Marilyn knows shit's gonna be hard af for her given that she can't reliably communicate with people outside of her sisters. Essentially, Marilyn's been trapped by her sister, so she has to stay on her good side. She can't risk Beldam's ire.

Watching Twitter explode with excitement over a screenshot of Beldam being seemingly transphobic in the Paper Mario TTYD remake is certainly an experience