I live in Colorado and my Mom has dementia. She made a Will out when she had her mind. Is her new will legal?

Is your brother caring for Mom in his home? She may have agreed with him that in exchange for said care most of her assets will go to him? To me that is not unreasonable. As to whether the new will is valid or not it would depend upon what her physician has said about her mental capability. Let us say that he wrote in her charts that she has severe dementia prior to the time the will was changed, then the will may not stand up to a challenge in court after she dies. That would be dependent of course on your getting a lawyer to challenge, and the lawyer getting access to medical records, and etc. It would be a costly process, and if you are not talking about huge amounts of money likely it would enrich only the legal system over all. A new will made by a rational person, or a person "capable of understanding her own actions" would, yes, trump and negate the old will.

It depends on her stage in dementia. Has she been determined to be incapacitated by her doctor? Bro cannot change her will, but she can if she understands the documents. It would be a call made by the attorney whether she understands or not. There are some unethical attorneys out there.

A demented person cannot make out a new will....it's not legal and won't trump the old one she created before dementia set in. Consult an Elder Care Attorney for guidance on the subject.
Good luck