Family is often fraught and moreso when a spouse or child gets disinherited. Perhaps these thoughts may prevent some pain.
Should you disinherit?
In my view, the best reason to disinherit a close relative is not to punish the person but because you believe the money would more wisely go elsewhere: for example to a kinder and/or more needy relative, or to a charity.
In any event, do be sure you’ll feel good, long-term, about disinheriting the person. Don’t let a temporary flare-up outweigh the longer-term picture. Of course, the decision isn’t permanent—As long as you retain sufficient mental capacity, you can change beneficiaries later.
Even if you feel there is a more worthy beneficiary and that you’ll continue to feel that way, still think twice before disinheriting someone. Even if disinheriting is justified, that person probably will feel hurt and thus—rational or not—possibly sue your heirs, likely costing them serious money, time, and stress.
Also, if other family members and friends learn of your disinheriting, they may criticize you. For example, some people believe “Blood is thicker than water” and thus, except in the most extreme circumstances, that you shouldn’t disinherit a child.
Mitigating the effects
Okay, let’s say you’ve decided to disinherit someone. Decide whether you feel it’s wise to explain your decision to the person. If so, there are at least two ways to try to get them to accept it gracefully.
One approach is to allow the person to save face. For example, “It was a very difficult decision. I recognize the good you’ve done, for example, (enumerate a couple things) but I’ve concluded that it’s wisest to give half the money to Mary because she needs it more than you do and the other half to Cold Spring Harbor Laboratory because they’re doing such important work on trying to cure cancer and other diseases.”
The other approach is riskier. It’s is to lay out the case, unvarnished, even though it will hurt the person. Occasionally, that—rather than sugar coating--is more likely to engender acceptance and even a change in future behavior. For example, “You have totally absented yourself from my life and when we’ve interacted, usually you’ve been cruel to me. Think how you behaved last Christmas, for example. We all make mistakes, but over decades, you have been so much worse to me than Mary has, so I’m leaving what would have been your share to Mary. If this is a wakeup call to you to try to be a decent human being to your mother, I am willing to try yet again to have the mother/son relationship we should have.”
If you’ve been disinherited
If you’ve been disinherited, apart from the financial loss, you probably are feeling hurt. And when hurt, you can feel like suing, even if in fairness, you are less deserving than is the beneficiary. Sure, you probably could concoct some way to rationalize that the decision was unjust, for example, “I did so many good things for dad. My sister was worse than I was.” But you might want to step back and list all the things good and bad that you did and compare them with what the beneficiary did. Besides, choosing a beneficiary is a judgment call. Doesn’t the person have the right to decide who should get his or her money?
Also consider that if you try to overturn the will, you and their heirs will likely expend money, time, and stress. Estate litigation can go on for years. And the burden would be on you to prove the will should be overturned. That can be difficult to prove.
An alternative to a lawsuit
If you’re already in a dispute over a will’s beneficiaries, before hiring a legal eagle, it's usually worth trying to talk it out together, perhaps with a mediator who specializes in inheritance disputes.
Ideally, good behavior should be driven by one’s intrinsic sense of right and wrong. But we all respond to rewards and punishments, so this discussion of will beneficiaries may be a useful reminder to all of us that our bad behavior may be punished and our good behavior rewarded.
Marty Nemko's bio is in Wikipedia.
 Note: The legalities are beyond my expertise—for example, the extent to which you can disinherit a spouse, which varies by state. This article addresses only psychological and practical issues.