So shouldn't But Cai have a younger sibling or something, he has been away from home for 8 years and now that he is part of the Chen family with good status very few would mess with his family, so having a second kid would make sense, especially since you said in this chapter that people in Xianxia reproduce like crazy. Truthfully I would love to see him have a younger sibling to spoil with excess resources from the Chen family, like Foundation building resources. If they could produce another genius that joined another of the 5 families or a sect their families status would definitely rise. The foundation improving the talent has already been indirectly confirmed, and even if the child goes to another family the bloodline supremacists would have to at least be nuetral to them out of respect for But Cai's status and connections with the various YM, so there would be very little chance of them getting abused.
Interesting Idea, I was mainly intending to leave Bu Cai as an only Child (At least for a longer time). However It may be subject to change, depends on how I am feeling. But even if it doesn't happen in this Universe, I plan to let him experience a large variety of different families and lives, as he goes from one to another, So he will have siblings at one point.