This is a grey area. There has been a lot in the media the last year or so and even threads on here, regarding adults with learning disabilities paying for sexual services. Should it be allowed or not, after all an adult has the same sexual urges regardless of there mental capacity.
It will come down to his mental capacity, his ability to understand the consequences (for example will he spend all his money not realising he needs it to pay for food bills etc).
It could be argued that the money payed was for the services. But how many services a month are we talking about. If it's twice then the money payed is obviously way too much. If it's 5 times a week it could be argued that's sufficient.
However, you know your brother. You know his mental capacity, his understanding etc. We don't. If you believe that he does not have the ability to make these informed decisions himself, that he has been taken advantage of, and that he is clearly vulnerable, then you need to report it.
As I said earlier, she may well be doing this to others too, who don't have a sister looking out for them. You are not only protecting your brother, you may well be protecting others.