Hi @Wcs1982 I’m so sorry to hear of your Dad passing away. Contact adult social care in your local authority - your sister needs to be assessed for appropriate housing, and for support with tasks like paying bills and preparing food etc. In terms of what sort of housing is available, it will vary between local authorities.
I’m a sibling too - my brother has severe learning disabilities. Sibs (www.sibs.org.uk) is a charity for the brothers and sisters of people with disabilities and has guides for adult siblings on a few helpful topics around managing care - you may find the ones on decision-making and mental capacity and managing benefits and funding particularly helpful at this stage. Have a look at https://www.sibs.org.uk/support-for-adult-siblings/guides/
Keep in touch and let us know how things go with finding appropriate care and housing for your sister. It must be a big change for you all at the moment and a lot to cope with. I hope you have supportive friends and family nearby too.