Let's assume the benefit is improved female fertility. If it's an autosomal trait, half of the carriers will be male, and lead to the vast majority of them refusing to have offspring. That would require the sisters to have a pretty spectacular bump in the number of offspring to offset their gay brothers. Indeed, since the trait is clearly polygenic, all the sisters do not inherit the trait equally, if at all. The sisters who do inherit it would have to be spectacularly fecund to compensate.