Welp, my curiosity was killing me but I wasn't sure I could take much more of what was going on in one sitting, so I committed the cardinal sin of temporarily skipping ahead, please forgive me.
Still, all is revealed. The friend really does exist and it really is her fault, and to top it off she's in the full blown "couples could never be as close as siblings" camp.
Also the other friend is just humoring them I think. I get the impression that she's got a normal or at least mostly normal relationship with her brother. The only real comment that begs much question at all is the kissing bit, but she could've meant when they were very, VERY young. Also I don't think she ever specified on the lips.
or she might just be a somewhat closet brocon. That's always an option too.