Published on 12, July, 2020
Sounds like you need really good storage, maybe get a local joiner to custom make some shelves and cabinets so as she can keep as much of her stuff as possible in the smallest space.
Maybe get your sister to watch something lke Stacey Solomons, Sort You Life Out, where people who live in overfull homes, stuffed with clutter, are challenged to get rid of at least 60% of it, whilst the team redecorate and create storage solutions. Maybe watching others have this good, difficult and life changing experience will encourage her to let go of some of her stuff and give you some ideas of how to support her.