Yes, your opinion is conjecture, while mine is a working hypothesis based on scripture, Catholic tradition, and Jewish custom.
Why can't you just admit the text itself and circumstances given is not enough to be dogmatic? We can't conclude Mary is everyone's mother because Jesus commanded John to take care of her. Just as I can't claim the "brothers of Jesus" were lousy half-brothers (or cousins in your case) who didn't even escort their widow and now childless mother (or in your case aunt)at a dangerous site like a Roman crucifixion. Yet a child of God, John, did show up and provide some sort of overwatch for Mary and the other women. Easy choice IMHO no? If I was in a similar circumstance and a blood relative failed to be at my mother's side in a delicate situation and my best friend was there...guess who I would choose to tend to my mother?
I just take issue with a hypothesis that is needed to explain away a firm word as in "until" or "till." The burden of scriptural proofs is at your end. Until means, well 'until.'