He feels guilty dating the other but he’s having feelings for the FmC that looks 8 years younger too.(although he knows that she is older than that so it’s a bit different)
I know this may sound like it may be a taboo in some cultures, but don't most people feel awkward in dating their sibling's boy/girlfriend's sibling? Could the easy way out would be is the two mc and fmc to date one another and get engaged so their siblings don't? I wonder if this is what the fate in this manwha is wanting?
Ah yes, he totally feel for her.
After all, they were already attracted to each other in the original story, but ended hating the other because of a setup.