If you weren't very privy to the younger sister's relationship details, and were only knowledgeable of the older sister's relationship (8 years together - everyone thinking it is about time he popped the question), would you think that the younger sister is only getting engaged because her older sister got engaged? The only thing you really know about the younger sister's relationship is that she has been with her guy for 4-5 years, and that the sister is 24, 1 year younger than the older sister.
Would you think the little sister "talked" the guy into proposing to her too? That the guys just happened to be planning around the same time and the proposal has absolutely nothing to do with the older sister's proposal? Little sister is trying to steal the spotlight/attention/whatever? That it was about time she got engaged as well?
Would you feel the boyfriend should have waited to propose? That a week is long enough for the couple to spread the excitement and there shouldn't be any drama