A song

You realised the perfect trigger, a song, you could make him set it as his ring tone while he's under and that way whenever he gets a call it would trigger a change, but he would also have the chance of hearing it anywhere and at almost anytime over the radio or from other people even if it is unlikely. You pick one of his favourite songs so there is also the chance he plays it himself until he notices that's the trigger. You decide to go with Mr. Brightside by the killers, it's always been a song he's liked so it should be on his phone already.

Now that the planning is all done you start to issue the commands to your brother "From now on whenever you hear any version of the song Mr. Brightside you're going to become a woman, you'll think that you are a girly-girl that loves to look pretty, you'll always be looking for cute clothes to wear and of course you wont be able to stand having anything manly in your room or wear any masculine clothes, you'll want to get the perfect figure and love fashion and all other manner of girly things, you'll be giggly and flirty and slightly ditzy, you'll also not get any pleasure from your dick whilst you are a girl but you'll be more than willing to explore your ass and of course you'll be attracted to men. You will keep acting this way until you hear the song again at which point you'll become your normal self again but you wont have any memories of what your female self did. Now before I wake you up you're going to change your phones ringtone to Mr. Brightside".

Whilst still under your brother picks up his phone and changes his ringtone all with a blank zoned out expression on his face "good" you say to your self having finally finished everything you needed to. There is only one thing left to decide, should you trigger him as soon as you wake him up and take him shopping with you today or, leave him at home and use your phone to trigger him while you're out and let his girly side decide what to do.