How do I deal with a controlling brother who truly loves our elderly mom, but who believes that he is the final arbiter of what's best?

Ask him to help you out by watching her, taking her to her appointments. Since her has not been actively involved in her care, he is probable unaware or in denial of her current situation. Please keep him aware and involved. If a situation should develop, there are family arbitrators out there that can help. If he is truly controlling, you may have to defer to him sometimes, or get a family arbitrator. There is this expression - "Do you want to be right or do you want to be happy?" You may have to give up your desire to be "right" in order to have a harmonious relationship with your brother. I wish you the best.

That depends on a couple of things. Is he the oldest child? Has he always been told that he's the 'man of the house now' kind of thing? My sister and I do most of the fussing with mom and dad. Our brother is a wonderful person, but he's always worked and she and I get the privilege of not having to, so it's us that go to doctor appts with mom and listen to her pain stories (which are totally true but exhausting). Well now my brother has retired from teaching and is available more. My sister at first got her panties in a wad when mom and dad would call the brother for assistance. But I told her to let him do it. He hasn't been as available as we have, and as far as I'm concerned this is a good thing. It's his turn I believe. Now whether this is even close to your brother problem I don't know, but consider how many siblings don't do anything to help. You and your siblings sit down and have a pow wow. You all want the same thing right? You all want to help mom right? So put your heads together and come up with a plan to make it happen. He needs to defer to your insight into what he's NOT aware of, and you need to let him get involved. Be happy, this is a good thing.