That's a hard situation, because you know that Mexican culture traditionally emphasizes the unity of family. I think the best thing you can do is to try to educate them on the importance of attending the best school that is financially feasible, while demonstrating the reasons behind it.
Assimilation is an inherent component of immigration, and although it is tough to accept, you can reassure them that visiting one another is always an option, Skype/FaceTime, phone calls, etc. will keep you in touch, and law school is only three years. In four years, you might decide that you want to move closer to them, or vice versa, and you will be in a much better position, both personally and professionally.