Is an brother entitled to prevent his sister from completing her studies after he prevented her from marrying a specific young man?

We apologize for not being able to fulfill your request, as it falls outside the scope of this website. We advise you to be patient and make abundant supplications (du'a). Allah may ease your affairs and grant you goodness and happiness. Goodness might lie in what you dislike, in accordance with His (the Almighty's) saying: "And it may be that you dislike a thing which is good for you, and that you like a thing which is bad for you. Allah knows but you do not know." Your brother has no right to prevent you from studying unless it involves something forbidden, such as mixing of sexes or if the field of study itself is impermissible. If his prevention is an act of injustice, you have the right to seek to remove this injustice through mediation.