Probably the author is trying to show us all characters as being well-rounded. So instead of Junichi being the "annoying immature ex-bf" and the FL's sister being the "hippy with good advice on matters in life" she is also showing us how the sister is coercive? manipulative? and giving us more background on Junichi. Yet I still can't like the sister- meh (lol not sure why I am getting so worked up over a manga!)
As for the ladies in the party lololol their inner thoughts had me laughing since I have also played polite and smiley while wishing for certain people to just scram!!! 😈