Maybe she's his sister (unlikely). Or his cousin (somewhat more likely). Or maybe he's evil enough to want to wreck Luke's legacy, but not evil enough yet to murder a small child in cold blood, so he hid her from Snoke.
Any number of reasons could have plausibility. It's definitely possible. Nothing else seems terribly MORE likely at this point (including the idea of her being some random unrelated to anyone and everything).
A few possible reasons, but I think the simplest would be: Imagine if she was highly gifted in the force (which seems evident) but also the youngest of Luke's trainees, the child we see in the flashback. Killing adults is one thing, but he may have drawn the line there. He simply didn't want to kill a child. So instead of killing her, he took her to a backwater planet, and told the junkyard dealer to keep her there for the rest of her life where she wouldn't cause any trouble.
But if this is the case and he's done this out of mercy or compassion, wouldn't he want to kill her as much if not more than Han? To rid himself of his light side/weakness? The moment he realizes who she is, surely he'd make it his damn MISSION to eliminate her.