I think the situation with Mel and Becca is so much more complex than people understand. I've seen people say Mel is being ableist or infantilizing towards Becca and I personally do not read this behavior as such— people are forgetting a couple of big things:
(1) Mel has significant trauma having to be Becca's caretaker for her adult life. She gave up hobbies and doesn't have an identity outside of Becca. She used to go to Renfaire, now she doesnt. She gave up her adult life to earn enough income to support herself and Becca.
(2) Mel is going through an extremely traumatic deposition where she's basically getting accused of causing intellectual disability for performing a life-saving spinal tap on an antivaxxers kid who was afraid of her kid getting autism. Shes overwhelmed, overheated, overstimulated and ever since she got out of the deposition she has been actively melting down/shutting down. On top of the fact that Becca, her only family and only close friend is being hospitalized
(3) Mel is also autistic, this doesnt mean that she can't be ableist but it also means she struggles to socially understand how others process the world relative to herself. She likely hasn't had any relationships and she tells Becca /everything/, she is hurt, confused and baffled as to how someone she thought she knew kept information from her. What do you mean Becca has a boyfriend?
The conflict here is not that Mel doesn't see Becca as a conscious being, able of making her own decisions— but Mel realizing her life is defined by Becca's and Becca's life is defined by herself.
My point is to say that people are focusing on this plot point as Mel-as-ableist or Mel-infantilizing-Becca when in reality what's actually going on is that Mel is actively struggling with her own disability, which she likely never got support for, which is the same disability she was the primary caretaker to her sister of. I don't think Mel was diagnosed as autistic as a child, because of how she thought her food issues were actually an ED. Had she had access to a diagnosis she could've understood this as her autism, but she was left to find different answers. I think she knows she's autistic, but likely won't seek out a diagnosis or help /because/ she is also her sisters primary caretaker and likely is afraid of the ramifications (legal or otherwise, though I doubt she has guardianship of Becca) of that label.