Where do I go from here...?

I have posted on the Forum a number of times about issues with my Mum’s care.She has Parkinson’s an is getting progressively weaker and vulnerable. Since August 2019 my sister and I have been caring for Mum-doing 3 'shifts’each a week from late afternoon until the following morning.This was working well for Mum as she had the support she needed;my sister and I were paid by Mum to provide her care for a year and then we would reassess the situation.
However, there have been problems.Although my sister suggested the idea in the first place,since the very beginning she has had serious doubts about her caring role with some dire outcomes,notably attempted suicide,violent verbal abuse toward Mum, and now the icing on the cake-she has found a job and virtually withdrawn her care for Mum to doing the shopping,'popping by’for half an hour occasionally and (at the moment) one ‘shift’ a week.
There was no prior dialogue between the 3 of us regarding this decision.Naturally emotions are very fraught at present with Mum,my partner and I.My sister is not communicating with me at all to discuss the issues raised-Mum is very anxious and doesn’t ‘want to upset the apple cart’ by trying to instigate dialogue with my sister,but I have suggested that we really DO need dialogue to try to sort something out.Due to my work commitments,I simply cannot offer any more care 'shift’wise at present;day-to-day care issues have been dumped on me as my sister is very deliberately keeping her distance.She has got what she wanted at the expense of everybody else.SO selfish.I have mentioned to Mum that,as her care ‘package’ has been affected by the reduction of ‘shifts’ a week that she may need to consider asking my sister for the money back so that she can use the funds to pay for care.Mum is reluctant to do this as she says it was a ‘gift’.NO! She paid both of us to care for her!
I have suggested to Mum that apart from trying to have dialogue with my sister, that it may be prudent to get in touch with Social Services again.Mum and I have both had assessments.Although I am aware that Social Services can only do so much,this situation is VERY dire as my Mum clearly does need help and care as she is a vulnerable frail adult.
I know that there is a limit to the guidance and advice offered on the Forum as it is aimed primarily at caring but I am wondering if anyone else out there has experienced similar issues and perhaps they could offer some advice,e.g,would it be appropriate to contact Social Services again?With regards to financial issues would it be prudent to seek legal advice?Thanks.

A bit of a minefield. firstly, prioritise mums well being. She needs to feel safe. Contact SS as things could deteriorate further before anything gets resolved. Your sister maybe using this as a get out of situation. Some people can not verbalise their own needs. You need to take the bull by the horns. Finances to a point can be sorted out later. Mum needs a care package that works for her now.

I think the line with SS is…

you have all supported mum in your own way. But things have now changed. SS are more than use to dealing with such situation. Care packages change constantly.

Contact social services to explain everything. Your mum needs help and she needs help now. Things will only get worse if you don’t seek help now.

Tell Social Services mum needs a “Care Act Advocate” URGENTLY.

I think what has happened here is that your sister had good intentions but did not realise exactly what she was taking on. I guess that the commitment of 3 shifts per week was just too much for her. The fact she has attempted suicide is an indication of her fragile mental health.
Don’t be so hard on your sister - she obviously couldn’t cope. At least she tried.
As for your mum - get outside carers in to help.