Hi Rafael,
Deciding when and who to tell is probably one of the hardest things we can do. You have to weigh all the consequences against the benefits. If you are regularly dressing for public then it is only a matter of time before you are discovered so you should probably tell your aunt before she finds out on her own. The timing of the telling is what you have to work out. As to your cousins they may surprise you as may your aunt. Careful though, don't spring it on anyone you don't have to until your ready, remembering also that if you are living full time they will find out rather you tell them or not(if they don't already know). People talk and word can spread rather quickly, especially if you live in a small or even medium sized town.
Fortunately for you, physically you are a girl dressing as a boy so people tend not to give it as much attention as a boy dressing as a girl. So you may be able to get away with it for awhile before anyone catches on. Eventually though people will start to talk. Use your best judgement, but if you are going to transition you have to tell.
Cassie