Good point. It could also be making a general statement "The big sister drinks alone so as not to disturb her little sisters"...a statement that applies to this situation, but isn't specifically about just this situation.Wouldn't the "-tachi" suffix be generally used to indicate its subject comprises of more than one person? E.g. "watashi-tachi" for "we" and "anata-tachi" for "you (plural)". In this case, "imouto-tachi" is more likely to mean "little sister (Kogane) and co. (Goro)" than "little sisters" (so no new fox siblings yet, unfortunately).
You'd see this more often in an allegorical sense, maybe like "the over-protective mother hawk watches over her chicks from a distance" (scene is Akane looking towards the hotel window with binoculars); of course, Akane is not the mother, Kogane isn't her daughter, and neither of them are birds, but it's a general statement being applied to the specific scene.
This is one of the frustrating parts of translating: there's often multiple ways to interpret it, and you can drive yourself crazy trying to decide which it should be.
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