"No, I am from your future," he answers. "Only by a month. But the... events are different." How can he explain those? He starts with one of the more recognizable ones: "Mo Xuanyu sacrificed himself for both Wei Ying, but only one of them retains his original form."

That's what he means, different. Sizhui could belong to his world, or he could belong to the other established world. Perhaps he could belong to a new one, that they hadn't seen yet.

But in any case, he is still family.