I was talking about this with a friend and saying how ridiculous it is to get offended over family, but he pointed out to me that "brother" is in quotation marks and this is apparently a reference to Moona's "brother" whom she accidentally called her boyfriend a couple times. So I guess when people put "brother" in quotation marks, what they really mean is "boyfriend."
... Granted it's still ridiculous shit, but hey, just thought I'd put that out there.