They're not actually
brother and sister in any genetic sense, but that's what they are to each other. (Very close and familial but not romantic at all.) And that is stepped up in this installment, when she loses ties with her biological family. He's always been an orphan but the family-substitutes (Sirius Black, Dumbledore) keep disappearing, leaving mostly Hermione, Ron, and the Weasleys in general.
I'll tell you re: Snape if you want to know but not sure where this thread is on spoilers. (I was shocked, shocked
when a friend on Facebook asked for no spoilers... who doesn't know by now? But then I realized that I managed to shut out spoilers until just a couple of months ago, when I read the series.)