When my wife turned 32 her younger sister(my sister in law) was 19. She moved in with us because her mother couldn't stand her any longer. She had always been a foul mouthed cunt and she never got any nicer as she aged. I wasn't happy about her moving in but I of course did not have a say in the matter. In the five years she lived with us she was black bred three times by three different men. Two are now in jail and one is just a loser.
So when my wife moved her sister Shelly in, she immediately amended our cuck contract to include her as a secondary cuckoldress. I was soon servicing her and her lovers in addition to my wife's lovers. Shelly loved her new role as she never cared for me and her evil soul enabled her to inflict much abuse to me which got her off I'm sure.
Knowing that once most men lose the urge to suck cock or eat cum as soon as they shoot their own load she insisted that I jack off on her feet before we'd scene. She loved to watch me gag down my own cum as I licked it off either her stinky feet or her likewise dirty ass. Then after everyone was gone to school and work, it was just us and her lover and I started my day sucking off her lover as she was not a fan of sperm herself.
"Suck my nigger's dick you worthless piece of shit" was her standard command to me. I did as told. She loved to tie me to the bedpost and would take up to an hour spitting in my face and in my mouth, often hawking "loogies" to amp up the humiliation.
If I even hinted at my displeasure she would often add some damn hard face slapping for good measure. She even knocked me out once and loosened a couple teeth. She also loved to kick me in the junk bringing me to my knees begging for her mercy.
When she'd work out she made me give her a tongue bath to lick the sweat off of her arm pits, belly, back and ass crack along with her feet of course. It seemed that her imagination never stopped as she continued to find new ways to humiliate me and gross me out in the process. At the start I was able to eat her pussy out only twice a month. The two days she was having her heaviest period flow, making me suck her tampons clean and then just eating her bloody pussy until she got off a couple times. She even got my wife to do the same. Evil thinks alike I guess. Eventually she let me eat her pussy almost daily. She figured out I was very talented with my tongue so after her lover fucked her good and hard she'd straddle my face, grinding her pussy hard on my mouth as her lover's excess cum would drip down into my open mouth and eager throat. She also loved to make me eat her asshole out when it was gaping from a good ass fucking.
As weird as it sounds, I started to fall in love with her and her ability to dish out the depravity. As much as I despised her I found myself craving her abuse. I cried when she moved out knowing that I would not be subjected to her daily cruelty. The wife never forgot the tricks her sister taught her and often makes me service her during her period and really gets off with a good session of face slapping and spitting.