You have a couple of potential issues. You ask about a Will and protecting your rights. Then you ask about someone using the parents' money. More information is needed. Is there a POA? Is there a Will? How do you know the Will was changed and what it provides? Was the Will and or POA prepared by an attorney? If so, the attorney will likely testify against you.
If you believe your parent is being taken advantage of, then you can contact the authorities. If you simply do not get along with your sibling, this will not help.
You may need to hire your own attorney to determine how best to proceed. You can petition the probate court to become guardian/conservator. There is no way to know what the judge will do, however, and your sibling might be appointed by the judge.
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You raise several issues. First, a will is not final until the person passes away and it is presented to the applicable probate office and accepted. If you think there is something wrong with a will or that it was forged, etc...., there are procedures to block probate of a will if done correctly. You need to consult a probate attorney in the count where the person will likely die, where their official residence/domicile is or where they owned land as to procedures to block a will. As to possible elder abuse, this is a serious concern and you can report to to the county agency on aging for an investigation. Also, keep in mind that whatever someone is doing as an agent under a POA is separate from the will, so there could be multiple levels of litigation.
The answers already posted offer some very good information and suggestions. I would only add that your parent's will can only be changed by your parent. It can't be changed by your sibling. If a parent changes a will based on false information or pressure applied by a third party, the will may be challenged. In that case the questions for the court are usually "Did the person who signed the will have the capacity to understand what he or she was signing?" and "Was the person who signed the will acting under the undue influence of a third party?" Capacity and undue influence are legal terms of art and have very specific meanings. You'll need to consult with an attorney to determine if either are a factor in your case.
However, as was noted in an earlier answer, the time to challenge a will is only after your parent dies, not before. All you can do at this point is begin to gather information that may support a will contest at the appropriate time.
If your sibling is using your parent's money, that may indicate your sibling is acting improperly. However, it may just indicate that your parent wants to help your sibling. A parent with Alzheimer's Disease may or may not be able to consent to making a gift to your sibling. It really depends on how much the illness is affecting the parent. There may also be a power of attorney in effect that allows the sibling to make gifts of the parent's money (although there is always an obligation to exercise a gifting power for the benefit of the parent). Depending on the circumstances, it may be difficult for child using a power of attorney to make gifts to show that the parent is better off after the gift than he or she was before the gift.
If there is a power of attorney in effect and if it's being used by your sibling, you may want to consider asking the court to order an accounting by the sibling. This can shed light on what the sibling has been up to.
I suggest you invest some time and money to meet with an elder law attorney in your area to discuss these issues in detail. Only then will you get a clear understanding of your options and a sense of how to proceed.