What should I do to prevent my step brother from selling a piece of land that belongs to our father?

You should consult with an attorney as to the details, and most probably will have to open a probate proceeding if your father is deceased.

That depends. Is your father still alive? If so, has your father given your step-brother authority (usually via a power of attorney) to sell the land? If your father is deceased, under what authority is your step-brother claiming to act? Is there a trust? Is there a probate estate? You or your father should probably confer with a qualified estate planning attorney.