Yes, financial aid is reliable, but it is not automatically given to you. You must apply for financial aid every year and Harvard re-evalutes your family's need every year. So long as your income (and your parents' income) stay about the same, the percentage of what you pay remains the same.
See: Application Instructions
. If you applied SCEA and filled out the financial aid forms correctly and on-time, you should receive an estimated financial aid award in mid-December with your acceptance letter. If you did not fill out the forms, you must complete those forms in order to get aid.