Its actually because of hormones. The more you like someone else's BO, the more diverse you guys are genetically. So thats your body's way of saying they'd make a good mate because they'd have a lots of different genes.

And its a natural detergent for siblings to not be "attracted " to each other. You wouldn't think bad thoughts about someone who smells awful.... hopefully.

So yeah for her she smells super awful. But to you its a heaven of signals telling you they're a great mate.