Your brother gives you the wand.
"thank you brother and watch as i make this date a bit HOTTER!"
You point the wand at both ofthem an watch as a faded version of. them appears to be getting sucked out of them towards one another.
The boys enters the girls body and vice versa.
The boy in the girls body is the first to awaken.
He looks down and notices he's in his girlfriends body.
He quickly runs over to the bathroom and you go in after him to see what happens next.
Then the girl gets up in the boys body.
She quickly relizes and runs to the bathroom next to you.
You tell your brother to go in there with him/her.
You try as best as possible to keep the wand hidden.