A Second Personality

You decide to give your brother a second personality, one that will come out whenever a trigger is used, and this new personality is going to be a woman. Not just any woman though, a complete girly-girl who always wants to look as pretty as possible and will always be wanting to go shopping for new cute outfits and decorating her room to be as girly as she is. You can't wait to see your brother 'willingly' feminizing him self only to come back and seeing all the changes and all the while knowing he can do nothing to stop it until he finds out what the trigger is and then avoiding it for the rest of summer, hopefully he doesn't find it until his new alter ego makes at least some long lasting changes, that would be hilarious. Now you just need to decide on a trigger, you want it to be something that you don't need to do your self but that wont happen so often that he switches so much the new personality can't do anything. You also don't want him spending all his time in girl mode because that way you won't get to see his reaction to everything he's done to himself. Or maybe that would be perfect, have him be girly all the time and only have a trigger that lets him see short previews of what's happening until an end of summer reveal, like maybe he only gains control when in his own room or something similar since you also want him to decorate his room which couldn't happen that way.

After a few more moments of deliberating and deciding on the particular details of the command you finally decide on a good trigger and how you want it to affect your brother.