Probably doesn't matter if he was married to her or not. A person can leave his estate to anyone he chooses and (except maybe for Louisiana) there is no legal obligation to leave anything to his children.
That being said, it looks like the will isn't valid.
According to GA statute 53-4-20:
- (b) A will shall be attested and subscribed in the presence of the testator by two or more competent witnesses.
So, if the will is not valid, and your father died intestate, then you and your siblings have a claim to his estate.
And that's when it becomes an issue as to whether she was legally his wife. If she was, she would also have a claim under intestacy. If she wasn't, she wouldn't.
Call the GA Dept of Community and see if they have a record of his marriage:
Regardless of that outcome, if you want to protect your (and your siblings') interest in the estate you (or one of your siblings) are going to have to go to GA and open probate under intestacy and petition the court to appoint one of you as representative of the estate.
But if you aren't willing or able to do that you will have to hire a lawyer in GA to do it for you.