Hi Clyde,
I can understand your concern regarding your sister. You say she has a care worker – does this person help with her money management? Does your sister have a social worker? This would be a good place to start as she may need someone to control there amount of money she has at her disposal .
With regard to the mobile phone, it is easy to put a gambling blocker such as “gamban” on her phone.
I’m not sure quite how much your sister understands about what is happening but it is important that everything is explained to her and that her opinion is respected on every decision.
Perhaps it would also be worth talking to your sister’s social worker (if she doesn’t have one you may need to apply for a disability social worker) to find out if she has other activities to fill her times if not perhaps they could also signpost her to suitable activities.
If you check the forums for people who gamble there will be lots of advice on there on barriers to prevent gambling.
There are also groups on here which you might find useful where you can chat to others.
Your situation sounds quite unique, and your concern for your sister is admirable – I hope my post helps a little.