siscon is just the oniichan thing of lolicon with a little sister that you see in a million anime and it's always creepy.
sorocon is something I think the bakemonogatari author made up based on this:
http://en.wikipedia.org/wiki/Chinese_marriage#Sororate_marriage. The dialogue is something like:
[A = guy, B = girl]
B: You always dote on your sisters, could you possibly be a siscon, perhaps?
A: No, falling in love with your sister...it's just a fantasy by those who don't have sisters.
B: Fine, you're not a sis-con. So you won't fall in love with your sister?
A: How could I!
B: That's right. You seem more like a Soro-con
A: Soro-con?
B: The abbreviation for sororate marriage, also known as inter-marriage inclusive of all sisters. It means marrying the older or younger sister of your wife whose passed away.
A: And why am I soro-con?
B: In your case, you'll most likely marry the younger one rather than the older one. You'll get your wife's younger sister to call you "oniichan" and then you'll kill your wife and marry her younger sister and still have her call you "oniichan" afterwards. Thus you can love your sister without loving your sister.
A: Now you're saying I'll kill my wife??
B: Well, because you're a soro-con.
A: I'd rather be a sis-con than a soro-con in that case!
B: I thought you said you wouldn't fall in love with your sister? Then will you fall in love with a lover who is not blood related?
A: Like I said...!!! Huh? I lover who is not blood-related?
B: Haha, you're so easy to tease. Small jokes get you so riled up.
A: These aren't small jokes!
B: I was just testing you now.
A: Why am I being tested? You mean you're not even going at me with everything you've got?
B: To use my full potential I would have to transform (henshin)
A: YOU CAN TRANSFORM!? Wow! I'd really want to see that! [sarcasm]
B: You make such huge reactions to everything, but you're always so petty. Still, regardless of how petty you are, I will never abandon you. I will cope with you and your pettiness until the bitter end.
A: Deep.
B: So, besides your pettiness, is there anything else I can help you with?
Obviously some of the dialogue rolls are lost in translation but the entire show is like 200 of those conversations non-stop with excellent directing and acting to pull them off in a fiendishly clever way. It's why Bakemonogatari is one of my favorite anime.