I can't believe you convinced me to come to this party, I really need to pee and the queue is so long...so I'm going to piss in YOUR mouth. That's the best thing a brother can do for his sister. You owe me this much! Go on, open your mouth, don't let any spill out! And since you were so good at it...I'm going to find any other opportunity to use your mouth as my own personal toilet. Oh no, brother dear! There's a terrifying spider by the bathroom and I'm desperate to go! Oops, I forgot to buy toilet paper, be a good brother and clean me up? Or even making you clean me up after fucking my boyfriend...and it's not just piss this time! Until eventually I get tired of making excuses, and tell you what your new place in life is: you ARE my toilet.