The kid's sister was killed at an unspecified time when he was 10, we can't assume it was after Sahara's death even though the story was told then. It might have happened before, and they're simply narrating it now.
But since that punk hair Beiyang graduate dude is present, the event had to happen at least after he rejoined Ditu. If I recall correctly, this was during the Merak/Tianxuan betrayal (Ding Yuan's god-slaying awakening) arc.
Now the kid is supposed to be 18. Has it been 8 years since then? I honestly didn't think so. Even if most important characters are ageless cyborgs or have enough money to stay youthful, there's stuff like Himeko's stepchild being still in university. Did he repeat the same class for 8 fucking years straight? (it wouldn't surprise me tbh 🤣)