Well, fiction has had several different ways to deal with time travel. Simply because of its popularity, we may be accustomed to the “Back to the Future” rules, where changes in the past affect the present. But other versions, like “Twelve Monkeys” or the French original “La jetée,” make it so that nothing can be changed, but everything works out as it always did.
I think it’s clear that this follows the latter rules. It was always as it happened. Ethan may have forgotten exactly what the girl he met looked like, but I’d have thought that the appearance of a unicorn in their lives would have reminded him, and her asking him about specific details would have clinched it.
Mike mentioned having a sister in one of the very first strips, where he introduced himself to B.D. In the February 27, 1971 strip, Mike set up a blind date between her and Mark, and the gag was that she turned out to look like a female version of him. But that was the first and the last time that we saw her, and later, of course, it was established that Mike’s only sibling was his brother Benjy.
Or Slylock Fox. I believe that the placement of the broken glass was in fact the clue to solve a mystery once.