If you actually think about it, while they really did look similar, each one of them still had their own personality. They think it only started at a certain point, but all of them were different already. Seeing how Yotsuba is known to have a bad sense of direction, we see that she's the one who got lost on her own. But things went into a bit of an overdrive with Yotsuba's jealousy, and their mother's death also causing a trigger as well.