I think the answer is simply that language doesn't strictly follow logic all the time (after all, the rules are defined by watching the language as it evolves, not the other way round).
In case the generic masculine ("Generisches Maskulinum") in German is already occupied with specific meaning - here: "Assortment of brothers" - Language needs to somehow work around this. In this specific case, it has chosen to do that that by employing the sisters. Apparently, there was simply a higher need during language evolution for a word denominating all the brothers in a family than for all siblings, so the brothers occupied that vacancy first.
EDIT: Dictionaries (and Takkat's answer) say that "Geschwister" used to denominate the sisters of a family originally - This doesn't change much of the argumentation, though: Language just considered the assortment of all siblings more important to have a name and thus re-used the sisters.
BTW: The generic for "Krankenschwester" is not Krankenbruder, thus somehow breaking the same "rule", just for other (historical) reasons.