I assumed it was because of this:

Hiyori’s tail isn’t really a “tail”–it’s an open link between her incorporeal and corporeal forms. In essence, it’s her soul.

I assumed that when her exposed soul touched Yato, she was able to tap into his dreams. It’s sometimes said that sleep is close to death, and dreams are messages that “cross over” between realms of unconsciousness and waking. When people sleep, in short, they’re nearer to the supernatural realm and have fewer “walls” up/are more vulnerable. Yato was in a more vulnerable state and Hiyori just happened to unconsciously “link” up with him via her soul, is what I figured was going on.

It is kind of tempting to think that Sakura and Hiyori might be related because they do look similar, but I think in this case it’s either a “we drew them similar just because we wanted to parallel their personalities” or “Wow, wouldn’t it hurt worse if Yato knew someone who looked like Hiyori and lost her in the past???”

I feel like, if Hiyori was a reincarnation or descendant of Sakura, it might have come back up in the story by now. It would be a bit odd to leave a dangling plotline like that for such a long time without any recalls to it.

What wouldn’t surprise me, though, is if Father’s freckle girl is somehow connected to at least one of the other characters (maybe Yato himself) somehow!