ANSWER - Your question is interesting as you reject upfront most of the suggestions you are obviously expecting to receive. Binding would keep the woman in your life but may reduce her impact, Banishing would potentially rid you of her but you have doubts this will work due to her relationship with your brother. I agree, I don't think either of these courses of action are best for this situation. What I think you need to do is look to open your brother's eyes to the true nature of this woman.
As I see it, either he is totally deluded by her in which case when his eyes are opened he may well leave her of his own accord or he is fully aware of her nature and loves her anyway in which case I'm afraid you're stuck with her. I'd certainly never suggest anybody undertook a working to split up a couple because they don't think the individuals are right for each other, such a working would be arrogant beyond belief. But what you can do is put him in a situation where he has all the facts to make the decision that is right for him.
So from the perspective of offering magical advice I think you should undertake a working to promote clarity and openness. There are a myriad of different ways to do this but if you are looking for something simple and straightforward, this is what I would do:
Get a picture of the woman in question and place it at the bottom of a bowl of water. Agitate the water so the ripples obscure the photo and (while holding something of your brother's - a piece of clothing would be ideal) stand and watch the water settling down while visualising your brother's image of the woman become clearer and clearer as the water becomes stiller. Visualise all elements of her true self as you meditate on the changing surface of the water. Allow all the things you want him to know about her to walk through your mind as you conduct the working.
You may wish to use some spell words, I would suggest something along the lines of
"Reveal who you are, reveal what you do
Let what you show be a reflection of you."
When you have finished the visualisation and the water has stilled so the image of the woman can be seen with perfect clarity, remove the photo from the water and wrap in your brother's clothing. Keep them safely hidden away together.
(If your brother does discover the true nature of this woman and can't find the strength to do anything about it, write to me again and we'll tackle the problem from a different angle.)