I have name changed as this is about somebody else.
Sorry if it's long but I don't want to drip feed.
My niece is 21 and has been in a relationship (lives with him he's 26yo) for about 2 years. They have just had a baby boy around 6 weeks ago.
Before the baby was born there were issues. Mostly centred around his insecurities resulting in him accusing her of affairs when she was 8 months pregnant and pressuring her for sex in the week leading up to the birth. He accuses her of not loving him because she refused to have sex with him.
I managed to get her to come an stay with me for a few days but she went back to him.
Once the baby was born he seemed to relax a bit and was showing himself to be the doting father and loving partner ( I was not convinced but kept my mouth shut).
However she has just text me saying she is upset as she refused sex last night due to tiredness and he argued with her that she is sleeping with someone else! WTF she has a 6 week old baby!
I have tried over the last few months to explain that this is not a healthy relationship that he is abusive. I have e-mailed her the info for woman's aid and talked to her regarding housing/money etc but she remains with him. When she is with me she seems to think clearly but as soon as she speaks to him or goes back to him she forgets what I have said or what he has done.
I have tried talking to her mother but she is just so wrapped up in her life and new relationship the only concern she has is that my niece will leave the abusive fuckwit and move in with her thus spoiling all her fun 
The sick thing is DN is currently awaiting a court date for charges she has brought against her father for child abuse (she reported it 2 years ago) which the fuckwit boyfriend is aware of. I have tried to get her to attend counselling but she refuses.
I live 200 miles away and I am going to visit her at the weekend but I am at a loss of how to handle this and what to do next.
I very much appreciate any advice and thank you in advance.