Unfortunately answers cannot always be found.  I am glad Jenny picked up on your post and I am sorry I have taken so long to write to you – I have had friends staying and been unable to catch up
Your situation is far from unknown on this site.   We have many CGs who have struggled though the illness and death of a loved one and used gambling as an escape from reality – it is hard for them to recognise they are taking that sadness as an excuse for them to indulge their addiction.  
I wouldn’t be writing on here if I did not know that the addiction to gamble can be controlled – even when the CG has had terrible things going on their life.   Your sister if only 54 and has years ahead to live either in control of her life or to carry on letting an addiction control her.   She can do it.    
I cannot tell you what to do but I do know that continually texting or emailing a CG shores up their belief that regardless of how much they indulge their addiction there is always somebody behind them  -and that is enablement.   I am not judging, I cannot, I did all the wrong things for all the right reasons for far too long.   I was always chasing, always willing to listen and therefore also vulnerable.   My CG lives in control of his addiction and has done for  years.  He doesn’t blame me for enabling him because I didn’t know what I was doing and his addiction was more than determined that I should not know – but he said that as long as I enabled I unwittingly prevented his ‘turn around’, his acceptance that he had to change his life.
I cannot tell you whether to tell her friends or not – but friends can unwittingly enable when they don’t know that an active addiction is in their lives.   CGs tend not to have friends – the addiction uses people and then moves on when enablement ceases.  
Your sister does sound in need of treatment – she is wrecking her life and hurting those around her.  Unfortunately unless she wants to stop enough there is little you can do.  What you can do is plant the seeds or recovery and hope they grow.  
If you click on ‘Resources’ at the top of this page and then in ‘Select’ click ‘World’, and ‘Gambling Help.   Scroll down to Gamblers Anonymous – 20 questions.   I think it would be good if you could print them off – tick with a bright colour the questions you ‘know’ your sister should say ‘yes’ to and then send them to her.   I think putting them in her hands could be confrontational and confronting the addiction sadly doesn’t do any good – the addiction likes confrontation as it gives it further excuse to gamble because nobody understands.    Your sister might screw the questions up and throw them away but she might be tempted to read them and she would see what is known by you and hopefully see that her behaviour is recognisable and treatable.  Perhaps you could include phone numbers and meeting of GA in her area or information on this site.
I think if it was me that would be the last formal approach I would make and then I would wait and see if there was any reaction.  
We can kid ourselves till the cows come home that we can save our loved one from the terrible addiction to gamble but I think it is important for non-CGs to realise that they cannot save another they can only save themselves.  
I wasn’t there and didn’t know when my CG changed.   He had phoned me still denying his addiction and I mentioned a rehab – it was a passing, fleeting mention but it triggered a reaction that continued until his eventual change of life.   That is why I believe so much in that small seed.   I gave him a direction to go in but he had to do the rest.
It is a sad fact that your niece is also not saving her mother and she would be better looking after herself.  Her mother is not deliberately bringing her down but her addiction will take her down with it too if she allows it.  If she succeeds then her addiction has even more wreckage to cope with and another excuse to gamble further.
It is common for a CG to mention suicide and I am not belittling their belief in what they say but it is the biggest manipulative tool in the addiction’s arsenal and leads non-CGs to feel responsible.   The only person responsible for the behaviour of the CG is the CG but they have to be willing to change their lives to take that responsibility – until then the addiction will blame anybody and everybody.
I hope some of this helps but please post again.    I was not living with my CG when he changed his life – he changed it because he had, had enough and enablement had gone.  
You have done well posting