Can a surviving spouse's rights override the terms of a Will?

Question Details:

My father and step-mother have wills. I was told by my father that I was the executor of his Will and he had made provisions for me and my children, along with my step-mother and her children and grandchildren. My father passed in 07/10 and I am his only child. I was told by my step-mother that because she is his surviving spouse, everything goes to her regardless of what my father's Will says. Is this true? And if it is true, can my step-mother change her Will l to leave everything to her children, instead of what my father's Will stated? Important Notice