@Bekka-m
I think it refers to how fake the bond between the MC and the sister is. Although she seems to have been caring for the MC, she has actually been behind the maids' schemes all along making her concern and their sisterhood fake or 'plastic'
Why is everyone complaining about Tita being the ML? First, the summary and cover(s) makes it very obvious as to who'll be our MC's ML. Second, y'all are almost 30 chapters in. You've read too far in to start complaining about the story now.
From the first chapter, I assume everyone in his office knows he has a wife. Why do all the ladies keep targeting him? That seems like it would be incredibly annoying...