She knows that dress makes her brother want to fuck her
Women know. You should never doubt their knowledge of human behavior. They don’t have to study or analyze anything, it’s like an extra neuron that they have and that comes with it from the factory. We kids are naive and we don’t understand these things, that’s why we fall very easily into their traps. She knows that that dress makes her brother very horny and then he wants to fuck her. So she doesn’t hesitate to use it the times she wants to have his cock inside her pussy. She even speeds up the process with a good blowjob until she feels firm.