Walking naked is her trick to warm up her brother
She is fat and a little ugly. Not more than a little bit, just to the point of not standing out much in front of most girls and that is what really makes her special. Because anyone who sees her will know that she will have zero expectations when it comes to choosing a male. He will basically take anything that is put in front of him and that is always good news. And also, she is clever, because the bitch starts walking around naked in front of her brother and that is her trick to turn him on. Then he takes out his cock so she can give him a blowjob and once he has it hard, he then proceeds to penetrate her pussy to make her enjoy it like a bitch. Which is how she likes it.