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8.
Slash (mainly) between twins.
1. Elladan and Elrohir, twin sons of Elrond of Imladris. From lotr
2. Weasley twins from HP
by brat July 03, 2004
 
9.
Twins having sexual relations. Capeesh?
Tom and Bill Kaulitz (Tokio Hotel) have twincest time on the tour bus every night.
by anonymous twc fan October 23, 2008
 
10.
When twin brother and sister have sex with each other. Something fans of Rosiel/Alexiel pairings from Angel Sanctuary write about in fanfics.
We're twin brother and sister who engage in twincest together.
by Michael & Michele September 07, 2005
 
11.
A suitable subject for Harry Potter slash fiction about a certain pair of Weasley brothers.
Fred/George = TWINCEST!!!
by Alianne September 14, 2003
 
12.
When someone is sexually intimate, or develops an incestuous relationship, with his or her own twin.
1. "The McPoyles from 'It's Always Sunny in Philadelphia' are definitely guilty of twincest."

2. Person A: "Dude, did you see those two guys going at it? They looked exactly alike!"
Person B: "Yeah, man, didn't you know? They're in a twincestuous relationship."
by NecroManzie September 27, 2009
 
13.
Stuff you say "woah, that's sexy" to.
Woah, that twincest is sexy.
by Kat-chan February 01, 2008
 
14.
When twins are in love with each other
Benji & Joel Madden (from good charlotte incase ya didnt know) - what a sight that would be...sadly the rumours arent true...are they...???!!?? ^_^ We'll just have to make do with the twincest fan fics!
by emo*ass February 16, 2004