Interesting

The idea of a girl daring her brother to crossdress is a good starting point.

Taking the idea further, the brother being dared to visit public places in women's clothes could add an exhibitionist twist to the story and could veer it away from **** if anyone's not a fan of that.

On the other hand, how about a guy crossdressing and getting found out by both his mother and sister at the same time. The two could have opposite reactions (One disgusted, one excited) or both feel the same, either way it could lead to some pretty good femdom.