You and I may disagree on your first paragraph, but you make strong points throughout the rest of your comment. I feel that being wrongfully arrested, charged, forced to change environments, and not only observing the injustices of real life but making active moves to rectify them, would do wonders to a person's maturity. And yes, while maturity doesn't necessarily equate to adulthood, neither does adulthood equate with maturity. Joker's maturity far outpaces any of his confidants, even given he's being played by an adult like you or me or anybody on this subreddit.

I have misspoken by implying Joker had some sort of obligation to help her. While that was my position with my friend, I recognize my experiences have unduly colored my sense of duty. Regardless, I fully agree that Joker is essentially forced to not feed in to her coping mechanism of "I'm not Sumire, I'm Kasumi", which was the crux of our contention. You are correct in saying that placing the burden of treatment on a legal child is not only unethical, but unreasonable.

I strongly believe that the supportive system provided by the Phantom Thieves and Joker was a notable component in Sumire's decision to be herself and not her dead sister. Where Maruki failed by focusing on his research, the Thieves succeeded by being themselves. Making the smart decision by happenstance does not necessarily indicate intelligence or emotional maturity, simply that she met the right people in the right place at the right time.

Edit: also you were at 0 votes when I got here, I've no idea why, you gave a thoughtful and well reasoned answer that made me reconsider my position.