ooh dang what a plot twist-
i also thought of that since the start. like what if they really are related and his bloodline has that ability to seal sukuna or at least rid of all his wrongdoings. but they didn't know when or who among the descendants would have to do it. i guess his grandfather had a feeling it was yuuji or it's a tradition to pass down that information but he wanted to be less elaborate as possible because he, in a way, doesn't want yuuji to be involved but knows that there's a possibility he could be.
Im surprise you people got to think that- i- OR- Maybe sukuna IS his great ancestor that- Uhm- Had to be alive so he can teach secret jujutsu to yuuji? Idk
What if, Yuuji and Sakuna are related?
Kind of his greatx10 grandfather or ancestor.
Remember what his grandfather wished for before he died. He wanted Yuuji to pass away with people with his side and not alone. What if its there Family's curse to die alone, due to the fact that they belong to a powerful evil cursed beings. Might explain why Yuuji who was tooooo young never met his parents nor forsee there death. (Btw, His grandfather died with him by his side because he was family yet not with normal ppl) Just my theory though... （￣へ￣）