I have been looking for an old story all over the place.
It is about a brother and a sister that find themselves separated in a new magical world. The sister gets turned into a catgirl and the brother becomes a mage powered by sex.
The brother sets out to find his sister and meets a bunch of monster girls to cast magic with.
In the end the brother finds his sister and uses her to charge up a massive spell to defeat the evil antagonist.
