Dear Ximena, 

If you are sure of your vocation, and have spoken to the sisters that you want to join, and they will accept you, I see no reason for you to put it off. It will be hard on your parents, but there will be many graces that come to them as well, and over time they may grow to understand and accept your vocation.

The idea of studying to be a doctor is not practical. Not only will it be many years for something you may not use as a nun, but you will most probably end up with significant debts to be paid off. I think that for you to take that path would essentially mean saying no to the vocation.

You should take into consideration the possible needs of your autistic brother. On the one hand your parents are probably still fairly young, so he will not lack care. His development will show how independent he is going to be when he grows up; not all autistics need someone to take care of them full-time.

I hope these thoughts help. Being as yet a minor you will probably have to wait for your parents' permission or your next birthday, whichever comes first, before making a definite move. Meanwhile keep very close to Christ in prayer, ask him to guide you and give you courage. And pray that your parents will get to understand the beauty and importance of your vocation.

God bless,