Buster Brown and Mary Jane: What exactly is their relationship?

I plan on making a post on my blog soon about Buster Brown, with somewhat
of an emphasis on Mary Jane, but I have run into a bit of a conundrum. From
what I've read, it's not clear what the relationship between the two is.
Some sources say she's his sister, which makes some sense given the strips
I've read that imply they live in the same house. Other sources say she's
his "sweetheart," which I interpret to mean girlfriend, of sorts, though I
guess that could be interpreted in various ways. Still others say she's his
best friend.

So which is it? Or was it just made vague and ambiguous and Outcault never
really said?

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