I am really sorry you are having such an awful time. She is clearly having a big effect on you, and I can see why.
This happened to me many years ago. People thought it was funny (the colleague in question cultivated an eccentric persona that masked the viciousness), and senior management told me to just deal with it myself. It was my mother who saw it plainly. She told me I was being bullied. I didn't like to admit that, but she was right.
My mother advised me to ignore the colleague completely, because any attention, whether positive or negative, was just feeding the behaviour. My god, it was hard. Made worse by the fact that at the time, I worked in a little room on my own. Colleague tried everything to get me to engage-involving others, bringing little presents, genuine (if unnecessary) work queries, mockery, invitations to things, little hurt voice, fake bewilderment, annoyance, requests for personal or professional help, contempt for my lack of manners, you name it. The record was a full 30-40 minutes in my room trying to get me to engage while I grimly stared at my computer screen in silence typing nonsense pretending to work, then about a hour of peace and a further 10 minutes having another go. After that day though, colleague pretty much gave up and I had my peace.
All these years later and the colleague is finally facing an incompetency procedure which should result in departure from our workplace before too long. I hope.
So I really get what you are going through. It is so intense. Over a decade later, DH still insists that colleague's name should not be uttered in our house, because if I start to talk about the whole thing I get sucked back in to all the anger and feel like crap for a good while afterwards.
I think my mistake was not to make the whole thing enough of an issue for others. I was junior and at first felt to blame, then stupid. I thought I had to solve it myself. And all the while, I was soaking up colleague's attention and annoying habits, so everyone else at work was spared. As far as they were concerned, that was a result, hence the reluctance to help me.
So, I get that you want to keep this job and feel vulnerable. However, you have got to share the misery so that others have an incentive to pull their socks up and do the right thing.
First of all, tell her straight but politely preferably by email but also in person that you can't spare all this time to talk and you need her to stop doing it completely. Tell manager that you are going to do so and ask manager to be present. Not because either of them will take any notice, but because you will document in your diary what you requested from manager and said to colleague and when and the fact that neither has complied.
If you go to the loo to avoid her, go in the Gents. Or the top executives' loo. Anywhere but where she expects to find you. The cleaners' cupboard if needs must. Explain to any person who asks that you have been left without all help and must use these desperate measures.
Another good tactic would be this: when she comes to speak to you, get up and say you can't talk as you have got to see X [X being a senior manager]. Then leave your desk and go into X's office and shut the door. Explain to X that as no one will help you with colleague's behaviour, which is increasingly have a bad effect on you, you are having to absent yourself to avoid it. As she follows you to the loo you can't go there, so will be in X's office for the next few minutes.
This has to become uncomfortable for the managers and not just for you. Only that will give the company the impetus to deal with it.
If necessary, deal with further approaches by saying you are late for an appointment. Then get your coat and walk out. Stay out for a while then go back. If anyone criticises you leaving during the working day, point out: (i) you are being harassed and the company refuses to help you; (ii) you can't get anything done with her yakking at you anyway; (iii) loss of productivity cannot be that important as they have failed to deal with your colleague's wasting of your and her time for a considerable period of time.
Finally, I am not encouraging complacency by any means, but while partner may be unpleasant don't take him entirely at face value-many people are good at verbal threats and bullying but do no more. Make sure, if it comes to a falling out, that you or a workmate mentions that your brother is a copper. Let colleague and her odious partner have a think about that.