Ayumi and Satori Joushima are indeed Strange Siblings. Whenever they will it, they are able to transform the other sibling into the opposite gender. Not only that, but they can even change each other's clothing to whatever they please!
Satori, the younger sister, absolutely loves this ability. She enjoys going places as a boy, and immersing herself into the "male culture." Ayumi, however, isn't as excited as Satori about their powers...
It's a shame, too, as they gain more control over their powers the more they use them. While Satori changes Ayumi at the slightest whim, usually to play with her own personal dress-up doll, Ayumi prefers to only change Satori when she asks or when she really makes her angry. That last one is rare, though, as Ayumi's usually too much of a nice guy to really get angry at his sister, no matter what she does to him.
So now, Ayumi can change Satori into a boy. It takes a lot of energy and it gives him a headache, but he can manage it. Satori on the other hand?
She can change her brother into a beautiful young lady, wearing anything she pleases, from anywhere on the planet, any time she wants.
Satori however doesn't mean to torture her brother; she believes that he enjoys being a pretty girl, and will often change him just as a reward. Ayumi is usually very quick to reject these sudden transformations, but sometimes she starts to be afraid that his sister is right. When he was dolled up like a bride, he couldn't bear to damage the beautiful gown, or even throw away the bouquet. Could he really enjoy being pretty and girly, deep down?
It doesn't seem that Ayumi will have a choice soon, as Satori has begun to develop a new ability; she can control her transformed brother's movements in any way she pleases, as she demonstrates to Ayumi for the first time, here.
The more fluent Satori becomes with her powers, the more control she has over her older brother. And despite not knowing it or meaning to, she's slowly turning him into the perfect little dress-up doll, day by day...