Sister or Woman?

I am the oldest child of four.  There is me. Then twin boys and another girl. My brothers are 23 years old.  The youngest twin is the outspoken and outgoing one.  He is currently dating a young woman who is 20 years old.  She is in school to be a nurse.  She works in a nursing facility.  She has her own car and own place.  She is able to buy herself nice things....

Instead she takes care of my brother.  She signed for him to get all of his equipment.  She put a phone under her name and gave it to him. When ever some comes down, he is driving her car.  She sends him money.

He is a dog.... He is cheating on her with another woman.  Using the phone that she bought him to call this other female. 

The only reason he got back with her was so that she would buy him the things he needed....now he is sick of her (but has most of what he needs) and is ready to dump her after she has put out over $8000 on him....

This burns my buscuits.  Not only is he using her... but he treats her like dirt.

Now part of me says to mind my business.  What they are doing is none of my concern. 

The bigger part of me says that she needs to know that he is using her.

But then, what makes me think that she doesn't already know and maybe that is her M.O.

Just because I won't be caught taking care of no not working grown ass man...maybe that is not her story.... she must enjoy it since they have been together for quite some time....

My heart just cries out.... How can you have so much going for yourself and go for this nonsense!

I want to talk to her....but I know my brother will be livid. 

So my question for the masses is: Am I a woman or a sister first??? Is it really any of my business?