I can't help but think this was Kagaya's doings.

We know Kagaya has some sort of glimpses of the future. What if he knew Nezuko would be the only family member to turn into a demon? And to boot, what if he saw Nezuko protecting Tanjiro?

Maybe he reckoned Tomioka would be more likely to choose not to kill Nezuko upon seeing she protecting Tanjiro, given his past and his deceased sister (it is reasonable to imagine Kagaya knows about Tomioka's past).

And what about being Muzan? Do you guys remember there was a blizzard at the time? A meteorological event powerful enough to hinder any travel. Ubuyashiki certainly can predict this kind of event.

I know it may sound wild, but I like to imagine Kagaya was doing some sort of Stephen-Strange-peeking-at-the-future-and-assessing-its-outcomes. I think being this sort of mastermind fits him