My grandfather with Alzheimer's is being taken advantage of by his brother financially. What can I do?

One option would be to obtain Guardianship of Grandpa. This way you or whoever becomes his guardian would be the only one that could legally sell property and that is only AFTER getting approval from the court.
Another option, if someone has enough influence with the Uncle is just to tell him to get lost and he will be reported as trying to take advantage of an elder. any items that were taken and sold could be reported as theft.

Question at this point is where is Grandpa living, is someone with him or is he alone? Sounds like he should not be left alone either for safety reasons and or you do not want someone being able to get to him to coerce him into signing any documents.
Also if he is not living in his home make sure it is secure and you might want to decide if the property should be sold with the money put into a trust (there are Special Needs trusts for this purpose) same with any vehicles. Any valuables should be secured as well.

If Grandpa has Alzheimer’s/dementia, he probably won’t be able to legally sign any documents. I would have someone trustworthy in the family apply for emergency guardianship. If Uncle knows you’re on to his ways, he may become secretive.

Because your grandfather has Alzheimer's he may be unable to understand that his brother is trying to take advantage of him and may lack the capacity to protect himself. As long as this brother is buzzing around I think someone in the family, on grandfather's behalf, needs to ensure that he's protected. This can be done with paperwork although with the Alzheimer's your grandfather may not be competent to sign legal documents. Another way is to tell this brother to get lost, that the family knows what he's trying to do and no one's going to let him get away with it. Safeguard grandfather as much as you can. Transfer his bank accounts to new accounts with a second person authorized on the account. Even this may not be possible because of the Alzheimer's. Launching a defense against the brother may be the only way to protect your grandfather.

Hi Drose
Does your GF have children or grands such as yourself involved with his care?
Does he appear vulnerable? Is that why your GUncle is so brazen? When you say family confronted him, who was that?
If your GF is in his ‘right mind he can sign his POA to the person who is willing to care for him and make health and financial decisions until GF death. This would prohibit GUncle from taking advantage of your GF. He needs a DPOA, a Will and and a DNR document if that is his choice.
Research these documents and help your GF make decisions to protect himself from his brother or others.
Ask any questions you have. You will get answers here.