wills and adoption

I am one of three adopted children. Our parents had no natural children. In 1995 my mother died and in her will- in the state of Idaho - she gave the vast majority of her estate to my brother and sister. I estimate that I received about 5%, or less, of an estate valued at around $500,000. A friend recently told me that there are laws that protect adopted children from such actions. Is that true? Is there anything that can be done abouit this?
Thanks. "Logowearus"