When people think about a will that is not valid, they often think about a fake will. Maybe, after a parent's passing, an heir drafted a will that gave them a far greater share of the estate than they got in the original. There is a clear chance for financial gain.
This type of thing does happen, but a fake or fraudulent will is not the only type of invalid will. There are many other ways that a will can fail to hold up in court. One such issue is if the person who signed it was pressured into doing so.
In all cases, signing a will has to be a voluntary action. The person must know that they're signing the will, they must really understand what it says and what it means, and they must choose to sign it without any outside pressure.
For instance, perhaps a caretaker spends a lot of time around the elderly person. They get them to alter the will in their favor and then threaten to stop taking care of them if they don't sign. The elderly person doesn't agree with the terms of the will, but feels they have no choice but to sign.
This rule also means that someone can't be tricked into signing a will, perhaps by being falsely told that it is a different document. If they did not know they were signing a will in the first place, they could not do it voluntarily, and so the will is not valid.
When you find yourself asking questions about the validity of a loved one's will, be sure you understand your legal rights.