So it's obviously hinted that the 'seo-in' in youngeoon's memory was seo-in's twin The tw...

So it's obviously hinted that the 'seo-in' in youngeoon's memory was seo-in's twin

The twin's death was probably connected to the kiss(I'm guessing s**cide based on how weird their mother acted with seo-in) that had gone wrong. This is just a theory from hints so far ╮(￣▽￣)╭