Where do I start? It's more of a AIBU or WWYD scenario!
My mum (50yrs) has been seeing a married man on/off for about 6 yrs.
When I say on/off, I really mean on/off.
He left his wife for her 2 yrs ago, that lasted a couple of weeks then went back to his wife. This has happened about 6 times since.
After the first 'break up' my mum confided in me that he had basically duped her into giving him alot of money. Her estimation is £20,000. She admitted this was the reason the home we grew up in was repossessed. I won't go into the reasons she gave it to him as there are many and she was partly to blame for being a mug.
The first time he left, his wife had the police looking for him as he left without contact for several weeks (he also has 2 teenage children). When they found him at my mothers address, he told them that he was too scared of my mother to leave so could they escort him home! 
The fucking weasel lied to get himself out of the situation.
Anyway after a while he managed to worm his way back in again.
All sorts of drama went on, arguements as he continued to feed my mother bullshit and she caught him out etc....I mean what was she thinking? Did she think that someone like that could change?
Another occasion he phoned the police I was there.
He called my mum while she was at my flat, they were fine, laughing and joking. Next thing, he put the phone down. DM heard his teenage DD pick up the other house phone and caught him out.
Two hours later the police turned up because he had phoned them to say she had been harrassing him.
I had to make a statement, DM had to show history of texts & phone calls to prove she was seeing him again.
He had used the police to dig himself out of it again. They gave him a warning for wasting police time.
This happened on one other occasion too.
While this was going on, things with DM became very strained.
DM has had severe depression & anxiety since her teens, has been hospitalised when we were children with a breakdown so as you can imagined she spiralled downhill. She was suicidal, having psychotic episodes, self neglect etc..
She is seeing a psychiatrist still, has behaviour therapy etc...
I have 3 DB's but as I am the only girl I think she takes most of her stress out on me.
She has screamed and shouted at me in supermarkets, threatened to hit me infront of people and also my DD......It got so bad I was regularly on the phone to her CPN as I didn't know what else to do.
My DB's have all had similar issues but not as extreme as I had had with her.
The last 6 months he has had no contact, DM has improved mentally, family relationships have improved. We havn't argued for ages over anything.....Until she dropped the bombshell that he's in tough again.
DM offered to have DD (7 yrs) while I had an appointment. Then casually dropped in that he would be there as they are talking again.
I tried very hard not to say anything there and then as I had to pick my jaw up off the floor. All I said was I wasn't happy for her to look after DD while he was about so would make other arrangements.
She came over today, I was dreading it as my DM cannot let anything go. She will still bring up things from 20 yrs ago and throw it in your face so naturally I was treading on egg shells.
She brought up the issue of me not wanting DD around 'The Dickhead'.
She was livid. I told her that I didn't want to get involved, her life is her business, I don't want to argue over him etc....
She then started to insult me infront of DD saying I was childish, who did I think I was, get off my high horse, whats he got to do with my DD etc..(with alot of swearing)
I explained again that im not with holding contact if she gets back with him, im saying that as DD's mum I don't want her or myself involved anymore.
She just told me to fuck off & stop telling her what to do, how pathetic I am and slammed the door, all with DD watching
She is lonely, depressed and I try to be as understanding as possible but I just don't want to get involved anymore. I tell her not to tell me things about it but she always creeps it into a conversation. Sometimes I think she does it to get a reaction out of me, other times because she has no one to talk to. She has alienated herself from friends & family for years.
AIBU to not want DD around this piece of shit of a man & WWYD in my situation, I really don't know. I am so angry and shaking about her behaviour just now I don't know if im right or wrong.
I don't think I can go through this with her again.