There is no way I can make this short. There's too much to it.
My sister was married to her ex for about 3/4 years, they have been split for 3 years and divorced for a year. I have never liked him, and I'm pretty sure he feels the same about me. They have 2 sons together, aged 4 and 5, she also has her eldest son who's 18 and my godson.
Right. Over the last 3 years, his behaviour has escalated to such a degree that I just don't know what to do. He sends my sister constant abusive texts, once she had 80 texts in a week - lets himself into her house, threatens her, and generally just displays really fucking odd behaviour.
If she doesn't respond in the manner he likes, he starts calling my mum. Telling her that her daughter is a bitch, that my mum is a shit mother to have brought up someone so disrespectful, she needs to do something about my sister and the way she behaves - at one point, he was texting my mum up to 10 times a day - she would switch her phone off and he would leave her voicemails. Sometimes he turned up at her house. As you can imagine, this was really stressful for my mum, in the end, I told her to save all the texts and think about going to a solicitor for legal advice. I have also advised this to my sister.
3 YEARS LATER - and this is still going on. And getting worse, frankly. He's been leaving my mum alone, but still being a dick in all the other ways I've mentioned.
Now. Last week my lovely mum was diagnosed with breast cancer
She is extremely shocked, and very scared. She's having a mastectomy in a couple of weeks. I've been callling her every day (I live 150 miles away), and after speaking to her tonight, she's really really down because my sister's ex has started up again. My sister's been telling her all the latest worrying stuff because 'she doesn't have anyone else to talk to' 
I just don't know what to do. My mum sounded so low tonight - she says she is at the end of her tether. I want her to concentrate on staying strong and getting better. And she's still getting all this crap laid at her door.
I'm furious. As far as I'm concerned, my sister should take care of this. It's gone on long enough, and everyone else ends up dragged into it, it's unimaginably stressful sometimes.
I've told my sister time and time again to call the police, see a solicitor etc, she'll go so far with it and then not follow it up. This weekend, she took the new man she's seeing into her local where there was a fair chance that her ex would be. Obviously now he's pissed off again and starting his crap.
I really want to speak to my sister about this. Our relationship is tenuous at the best of times, she is bipolar and very difficult to approach. If I speak to her, I run the risk of being cut out of her life. But the way I feel at the moment, it's a risk I think I'm willing to take. My poor mum. She just can't afford the headspace atm for this.
Does anyone have any advice?