"Your siblings are drinking wine."
Translation:Twoje rodzeństwo pije wino.
When assurting value it must be compared to somethig. Thus, it is understood that for most non Polish speakers- Polish is a tricky language to master. In depths why not try to break up the logic through self defieting retoric: given that most of the world does not speak Polish or Czech, The Polish language is rather hard to learn (compared to most people).
You are technically right, although for example Wiktionary doesn't even take plural into consideration. But "rodzeństwo" may refer both to "one brother" and "seven sisters and five brothers". It's rather a collective noun. And usually it's the plural brothers/sisters interpretation, because otherwise it's easier to just say "siostra/brat" and not "rodzeństwo".