Praise be to Allaah.
You have done a good thing by taking care of your brother; this is an act of kindness towards him and towards your parents. But marriage is a Sunnah, one of the ways of the Prophets, which Allaah has prescribed for His slaves and He has created them with an inclination towards it. I think that you have the choice of a number of solutions, as follows:
If you can afford to hire someone to take care of your brother, this would be good. Or you could look for some good people who would sponsor all or part of his care.
Or you could get married and have your brother come and live with you, so that you can carry on looking after him.
If you find someone who agrees to get married on condition that you stay with your parents in order to take care of your brother, and your husband comes to visit you in your house or you go out to visit him sometimes, this may be a good idea.
If none of these solutions are easy for you, and you feel that delaying marriage will not cause fitnah (temptation) for you, there is no sin on you in that case. But if you fear that you will be subjected to fitnah, then it is better for you to get married, because this will protect your religious commitment, and serving your brother is not an obligation upon you, rather it is voluntary good work.