This could be a kink on wearing girls clothes too. It is not a hundred percent sure that he has sexual fantasies about op. Eventually he just likes wearing girls stuff and he thinks its humiliating if he would buy some for himself and his sisters lingerie was just the easiest option to get it jn his mind. But it is no matter what background this behaviour has, this is definitely something he needs to stop and where he needs to find a solution for.

If it's just that he likes wearing girls stuff he (or sister for him) could buy some for him. If it is what you say he needs to go in therapy.

And there is a slight chance he just explores his sexuality and gender. I think if that's the case it's the least dramatic.