Try the National Centre for Domestic Violence.
http://www.ncdv.org.uk/
They are a charitable organisation and used by quite a few Police Forces, including the one I work for. Brilliant People.
Although your relative appears not be be subject to violent behaviour (though this is difficult to see since abusers often hit people on parts covered by clothing and/or the victim will never admit to it), the behavour is without doubt Domestic abuse.
For those that say 'why doesn't she leave him' it isn't that easy. People are not stupid. Years of conditioning often wears people down and breaks their spirit.
Having dealt with many vicitims over the years one of the most common things they say is 'he's lovely sometimes' or 'he can be so caring' - oddly they love this part of the offender and it can be a major factor in not leaving him. It is odd, but it happens. You have to bear in mind most of the relationships were fairly normal at the beginning and to give up someone that you once loved is a very difficult thing to do.
Victims can be fearful that the Offender will never leave them alone if they leave. They are often scared to make that jump to safety into the unknown. There are a multitude of reasons why they don't leave.
I would go to your local Police Divisional HQ and ask to speak to someone from the Domestic Violence Unit. They will help.