Isula Home from College
Well they're both college age, so adults. The sister about a year or so younger then the brother. Both went to college and discovered the fun of BDSM, from different angles of course. One day as they're both home from college the sister is in the shower, she's sneaked in a vibrator. She has her hands loosely bound with the rope from the soap on a rope or the shower spounge thing over the shower head, and a rag balled up in her mouth. Vibrator of course going in her sex." She says with a wink, "That's just a start for that, but you can guess what happens next when the brother discovers his sister's love of submission and bondage.
Another aspect I thought could be fun, is that one time they're alone at the house for lunch and the brother decides to get the sister into a tight arm binder, a ring gag on a training harness with a blindfold and under the table. They both know that someone else in the family will be back soon so she hesitates. But is convinced when the brother says that the table cloth will hide her. After she's in place the bother removes the table cloth, leaving her naked on her knees under the table, and has lunch with his cock in her mouth. Of course that's when one of the other family members comes into the kitchen, sees the two, and see's nothing wrong with it, doesn't even comment on it. What the sister doesn't know is that the house is a bdsm house for the most part, just not telling the kids till they're old enough.
I thought of another one that could be interesting too. Maybe going to a rock concert that the two like. The group is BDSM based so the fans often wear leather gear. The bother decides to help her with her clothes. Thigh high bondage boots. Tube dress that zips up the front. And he convinces her into a matching arm binder, with hobble chains. Of course some girls are wearing cuffs, but nothing like the sister. There is a bit of a competition to see who could be the most wild, and at this point the brother unzips the tube dress, and leaves her naked, infront of all their friends. A nice gag could also be added.