The promise does not matter. What matters is the will if there is one and if not, it would go to her children anyway. Have you browsed through the information in LawInfo's Free Legal Resource Center to learn more about your issue yet? See:
http://www.lawinfo.com/consumer.html. You can certainly try to speak to a lawyer to determine what legal options may be available, but most will tell you the same thing. In the meantime, you may be able to learn more on your own. Search the "Free Legal Resources" tab, or browse the Consumer Resources. Good luck.