It is generally difficult to challenge a Will, and that is a good thing. About 65% of the population has no estate planning. For the people that take the time and spend the money to actually meet with an attorney and get their estate plans set up correctly, it should be *very* difficult to overturn such documents.
The courts are available for situations where someone CLEARLY took advantage of a testator in a weakened or compromised state. But even people in such conditions, (and many would argue ESPECIALLY people in such conditions) should be able to plan their estates. This is one of the reasons why having an estate planning attorney involved in the process is so important. The attorney can interview the client and make sure that his or her intentions are reflected in the documents. The attorney is then also available to testify in court as to the decedent's wishes. A good attorney will lessen or eliminate the potential effects of undue influence.
Once documents are properly set up, it should be very difficult to set them aside. That is generally the case. On the other hand, in our justice system, just about anyone who wants to is entitled to their day in court. If they feel they have a legitimate challenge, it is preferable to have it aired in court as opposed to dueling in the streets.
These challenges always depend very heavily on the facts of the case. There is no reason why someone in the initial stages of dementia cannot properly execute a Will. Even someone who is in the end stages of Alzheimers can have good days, when they would be capable of signing an estate plan. But it is not the population in general, that matters. It is your father and his medical condition that is relevant. If you have evidence that he had no idea what he was doing, or that your sister strong-armed him into this, and you can overcome the burden of proof, then by all means, you should meet with a lawyer to discuss how to proceed.
*** LEGAL DISCLAIMER I am licensed to practice law in the State of Michigan and have offices in Wayne and Ingham Counties. My practice is focused in the areas of estate planning and probate administration. I am ethically required to state that the above answer does not create an attorney/client relationship. These responses should be considered general legal education and are intended to provide general information about the question asked. Frequently, the question does not include important facts that, if known, could significantly change the answer. Information provided on this site should not be used as a substitute for competent legal advice from a licensed attorney that practices in your state. The law changes frequently and varies from state to state. If I refer to your state's laws, you should not rely on what I say; I just did a quick Internet search and found something that looked relevant that I hoped you would find helpful. You should verify and confirm any information provided with an attorney licensed in your state.
It is probably prudent to meet with an attorney regarding this. I would add, however, that answering this question is not just a matter of deciding that you have a solid factual basis for proving undue influence; it is also litigation against a sibling. This means two things. First, litigation can be expensive and protracted (i.e. thousands of dollars may be spent to have questions answered two years from now). Second, there is an emotional toll for resolving disputes with family members in court.
Disclaimer: This answer is provided for informational purposes only. This answer does not constitute legal advice and should not be relied on. Legal advice can only be provided after consultation with an attorney with experience in the area in which your concern lies. This is so because each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and/or documents at issue. This answer does not create an attorney-client relationship.