noun. Sis-to-sis ew, or bro-to-bro gah, or sis-to-bro gak.
Related terms: cousincest, dormcest, housecest, dogcest, horsecest.
Real citation: "And the fanficcer suddenly decides 'Married With Children' siblingcest fanfiction is the REAL DEAL, walking away from the story, ignoring the clamoring cries of those left behind that run along the lines of, 'Now what!!??'"
(Mad-Hamley, Oct. 13, 2004, http://www.wiffy-dreamverse.com/modules.php?name=Forums&file=viewtopic&p=1398)
Made-up citation: "Does anyone have a copy of the siblingcest episode of Family Ties?"