jjk chapter 149 spoilers below
I'm not sure how to feel about mai's death honestly, I think it would sit better with me if we had seen more of her and her relationship with maki after the goodwill event, but as it is their reconciliation felt like a rushed mean in order to push maki's story further.
it's not out of character for mai to sacrifice herself for her sister because there are no other choices left, that's what she's been enduring her whole life. she's been a passive victim of her family's beliefs and her sister's rebellion and she died as such.
one could say that this time she took an active role: she chose to side with her sister, she chose her cause, she understood her rebellion. but did she really? when did her views change? what caused them to? because as it is it just looks like a final sacrifice out of love.
"you are me and I am you" is a lie in the same way "master tengen is me and I am master tengen" was a lie. a person should always be more and bigger than a bond, but unlike riko, mai never had a chance to discover that. maki is maki and mai was what maki needed her to be.
mai's sacrifice isn't powerful, she didn't take agency back. she simply did what was expected of her.
mai's death is a tragedy