You mean like a cosmic hiccup or something that made that change? nope, nothing more than the passage of time. Even when Stan and John Romita introduced the idea that Sharon and Peggy were sisters, it was a bit of a stretch timing-wise, and the passage of time has made it completely untenable. But making Peggy Sharon’s Aunt allowed for the essentials of the relationship to be preserved without changing a whole lot else and still fitting into the chronology of the Marvel Universe. (Eventually, even that isn’t going to work, and something else is going to need to change–but given that Peggy is now deceased, it’s not likely to come up a whole lot either way.)