If my father remarries is his will that was made when my mother was alive invalid

If your father's principal residence is in Georgia, then any Will he made while your mother and he were still married likely does not contain any provisions contemplating the possibility that he would get married at some time after he created the Will. If it doesn't, then when he remarried, his new spouse would automatically have become entitled to a minimum 1/3 share of his "probate estate" assets if he dies with that old Will still in effect, since she would be treated as if he died with no Will and would receive an equal share with each child (but no less than 1/3). The rest of the Will would then normally continue to remain in effect. His new wife will also have the right to make a claim for a year's support against the probate estate (unless she waived it in a pre- or post-nuptial agreement) - this could allow her to take all of the probate assets if she asks for them and is either not challenged or wins the challenge. She will also be entitled to any assets on which he makes her a joint tenant (joint bank accounts and certain deeds, for example) or names her as a beneficiary (IRAs, life insurance), and to certain assets subject to ERISA (401(k) plans, for ex.) even if he doesn't name her as beneficiary. If he has his principal residence in some other state, that state's laws would apply to the Will, and it could have become completely invalid. Other states also have a set minimum share for a spouse, rather than year's support. Your father really needs to see an estate planning attorney and make sure his estate plan will work the way he wants it to. Remarriage is a very big step.