I think it would because he would have had other reasons to continue living in the “new” life but the sis serves as an attachment and sort of obligation for the old body/Sihyeon while there could be other attachments. My complaint mainly is that the author did not flesh her character out well other than being a happy sibling with a tragic story/illness that needs her “bro” still and acts as the light of his new life, so he keeps living. We also got flashbacks recently (spoiler?) of how the og Sihyeon was pretty much at his lowest and how her character wasn’t enough (!! the main point is her alone isn’t enough) for him to keep going.
And don’t get me wrong, she is far from useless. It’s just her specific role could be filled by others imo at this current stage, when I think the author can give her more character and purpose. The sick sibling trope wasn’t done well here. Her character def has potential. Who knows, maybe the author gives her more of a spotlight in the future to shine.

But I think the story would have still worked in having Sihyeon continue to live after everything even if the sister character was not included or changed. I'm also glad about the current Daejun and Sihyeon relationship (like progress and acknowledgement!). I hope Jaeha suffers.