These are very good and difficult questions. A lot can depend on the elder law attorney with whom you work. Some are stricter than others about exclusively representing the elder parent. In his confirmation hearings to become a justice of the Supreme Court, Louis Brandeis suggested that in certain instances he had been the "lawyer for the situation." In short, a lawyer can represent multiple parties with differing interests as long as there is full disclosure and informed consent by all parties.
You should be able to work with an elder law attorney to work out a plan as long as both you and your brother are fully involved. The lawyer would have to meet independently with your mother to make sure that she is comfortable with the situation, even if she can no longer understand the finer points. But there may be attorneys who are less flexible. We would recommend interviewing a few who have offices near where your mother lives and choose the one you feel will work best with your family. To find an attorney near you, go here: https://www.elderlawanswers.com/elder-law-attorneys.