It's your 18th birthday and you are in your room. your big sister comes in bitchy as usual telling you that she has some special plans for your birthday and she wants to show you but not quite yet. She toys with your mind telling you this will be your last special day and shes been waiting for it for such a long time. She starts to get naked and tells you that she catches you watching her sometimes and knows you like her boobs. She lets you touch them briefly before getting naked and removing the sheets to find your massive boner. She tells you that she can fit you completely inside of her and starts to ride you. You don't believe her and shove her all the way down the shaft which leads her to explaining that she wants you to get her pregnant and that you are going to be shoved completely inside of her. She starts with your feet and continues all the way to your head mocking and explaining further that mom was already in there and she was to be the new mommy of the house. Once you are shoved inside she plays with herself telling you to fuck all the parts of her body in there (uterus, fallopian tubes, etc) to make sure she gets knocked up, She gives you joi making you cum in her load after load until she is sure she is pregnant, melting mom into slut slush and you eventually into cum serving your only purpose.