Bully aunt - what to do?

I have an aunt who I suspect has some sort of personality disorder, she's viscous, belittling, obsessed with money and an over all awful person. Every encounter I have with her consists of subtle and not so subtle digs, judgmental comments and nasty remarks. She's been this way as long as I can remember. She bullied my mother from when they were small and continuous to belittle her, says hurtful things, starts conversations with the intentions of getting under her skin or to get a rise out of her through bringing up certain topics that she knows upsets my mother and makes judgmental and vile comments about me and my siblings. She genuinely enjoys causing hurt. She has this smug look on her face when she's saying something nasty and you can hear the pure glee behind her voice when she's making digs.
I avoid this woman like the plague, I avoid going anywhere where she might be but sometimes seeing her is unavoidable.
Every time my mother speaks to her she comes away upset and angry, ive told her not to engage in conversation with her, ignore her and don't get into conversations regarding issues that she finds upsetting - I've told her to walk away from her when she starts to get nasty but she doesn't listen. Every time she sees my aunt she's friendly and desperately tries to get this aunt to like her, if this aunt say's or does something nice - which is rare and only because she wants my mother on her side for one reason or another or she's looking for something, my mother comes and tells me about it, delighted with herself that her sister showed a little bit of respect towards her - this always turns out to be a tactic by my aunt and my mother falls for it every time, she's even reduced herself to belittling me along with her sister in an attempt to get her to like her.
My mother has a tendency to act like her sister (although not as bad) until she's stood up too, ive been at the wrong end of it a number of times, she bullied me for a number of years when I was younger which resulted in me having a suicide attempt. - This goes to show how nasty they can be but that said my mother has never been as vile as her sister, she's on her own level.

I understand that its her business, I don't interfere - Ive only ever told her to walk away from my aunt/avoid her when she's been upset after an interaction with her. What gets to me though is how defensive my mother is regarding her sister and how fearful she is of saying anything negative about her - she fears their mother is looking down and will send her bad luck for talking negatively about my aunt.

Now my aunt is currently in the process of destroying something my mother has been talking about doing for awhile now, she openly admitted this to myself and my mother in a previous interaction. A few days after this she showed up at a family event on my fathers side, when I saw her I couldnt help but give her blank stare and turn my back. She had no reason to be there, I asked my mother why she turned up but my mother became very defensive and said she was glad she was there. - I didn't get into it with her just said ok and left it at that.

Everytime this woman belittles, insults, or takes digs at me I remain calm and leave the room but this is becoming increasingly difficult for me to do and one of these days I am going to tell her exactly what I think of her but what really gets to me is how much of a doormat my mother is when it comes to this aunt. I wouldnt care only its beginning to effect something my mother has hoped for and has been working to make happen, her sister is actively interfering just to cause annoyance and hurt.
Im just wondering what others would do in this situation?