The legalities in the emotionally laden situation you describe depend on one thing: your father's actual mental competency.
For better of for worse, the law regards marriage as a
contract"”and requires that both people marrying must be mentally competent"”that is, they must be able to recognize and understand what they are doing. This is not an exceptionally tough standard to meet"”and many people who have Alzheimer's or other limiting conditions are considered legally competent to marry.
If you believe your father truly lacks the mental competence to enter a valid marriage, you could oppose the union or challenge it on this ground.
It might then also be logical for you to petition to have someone appointed your father's legal conservator or guardian so that there is another person to care for his finances and personal well-being. This is especially important if you have good evidence that the woman in question is limiting his access to needed medication or acting in a way that may endanger your father's health. But know that you might also risk alienating your father, if he is cogent, for good once you take this action.
If your father does have the legal capacity to consent to marriage, then you face swallowing a bitter pill: Wish him the best and understand that he might choose to spend the rest of his life wed to a woman he has been dating for 20 years, even if that wouldn't be the person you might choose for him.
If he has sufficient legal capacity to marry, he likely also has the capacity to make a will or a trust or other estate planning documents indicating how he wants his property distributed at death. Talk with him about your concerns, leaving out any particular bitterness you may feel about the woman he intends to marry.
It would not be unusual, for example, for a person in your father's position contemplating another marriage to enter into a prenuptial agreement protecting property he intended his children to inherit, or to set up a trust or will designating specific property for them. Perhaps you could convince him or the merits of one of these approaches in the name of family harmony.
But brace yourself for another potential bitter pill: Your father's wishes for his property may have changed over the years. If so"”and that means you and your siblings get a small share or nothing at all"”you may have to live with that possibility, too. Your aim might then become to make the best of the remaining time you have together.