From the context, quite obviously a girl, since her sister teases her about sisters+girl-on-girl-action. Plus, she looks like a girl. To me, at least. Plus, bathing together is more common for same sex siblings.
Just for the record, the author posted more recently on Twitter saying (as best as I can tell) this is just an April Fool's gag and doesn't actually have an older sister like that.
From the context, quite obviously a girl, since her sister teases her about sisters+girl-on-girl-action. Plus, she looks like a girl. To me, at least. Plus, bathing together is more common for same sex siblings.