You were Giyuu's younger sibling...he was a year older than you...

After the fight with Muzan...the corps disbanded...Giyuu couldn't get over Shinobu's death...he married someone else to get his mind off things...he had kids with her but he still felt empty...

You decided to visit them...them...his wife greeted you happily but you resented her...you wanted Shinobu chan to be his wife...

(Let's call his wife Shiyue)

"Oh... sister it's nice of you to visit..."