Pretty much everybody has a bone to pick with the Demon sisters getting an absolutely incredible introduction and almost immediately becoming a Goldfish Poop Pair. But lots of Western cartoons have similar problems (Him, for example, fluctuates between "silly fruity bad guy" to "absolutely terrifying Satan analogue"), and the show is supposed to be a send-up of Western animation...
Lots of Episode 12 was foreshadowed when you look back.
In a Casino, before Brief was kicked out, you notice he has a lot of money. He also buys Panty's panties for $200.000 when they are sold on the internet. His dad's rich.
Which probably explains why she was okay with falling in love with a ghost.
There's one piece of Fridge Brilliance that's only fully apparent when you listen to the OST. While "Fly Away" and "Champion" are the good guy themes, their lyrics are about betrayal, while "Theme for Scanty and Knee Socks" and " Corset Theme" are more love-centered. Stocking's Image Song is about love.
The ending theme shows the characters in deadly situations while "Fallen Angel" plays. Taking into consideration that some works of fiction depict angels as the souls of the dead, one could interpret the ED as showing how Panty, Stocking, and Chuck died. Of course, the series itself gives no indication that its angels were ever human, but...
The Ghosts represent the grudges of the dead. So what was the 'grudge' of the Ghost that pretended to be a child prostitute?
Turns out they're the incarnations of a prostitute and her pimp who were killed by a wolf while having sex and a picnic in a forest.
In "Death Race 2010" Panty discards her underwear, whining that she can no longer wear it thanks to the monster of the week. A few episodes later, in 'High School Nudical', they're up against a Ghost that eats undies. Garterbelt tells them that it's the grudge of underwear that was thrown aside. Think about that for a sec.
If the OST artwork is any indication, that may in fact be the case.
Not really, if you think about it. Demons are (or at least can be) simply angels that fell down to hell, so an angel becoming a demon or one sister out of a pair of sister angels becoming a demon isn't really all that impossible.
Maybe Stocking's big entrance in 13 was her getting kicked out again...
In "The Buzz of the Beehive", it's implied that Panty does not know what a condom is. You have to wonder, with as much sex as she has on a daily basis (and her seemingly non-existent standards) that she would be a veritable cesspool of venereal disease...however, according to Word of God, "angels don't get fat, grow old or get sick". If the girls don't return to Heaven they'll become humans and be unable to maintain their lifestyles without some serious side-effects, which explains their motivations.
Then again, it could have simply been a case of her not paying attention (she mistakes it for gum). In both a later episode and the tie-in manga, it's implied that she does have some condoms lying around.