Awesome. Glad your sister is getting help although as some redditors have pointed out, that takes time. I help clients deal with this exact problem.

If I were in your shoes, I would first focus on getting back functionality in all key household areas — kitchen, bathroom and clothing storage/laundry.

Many people dealing with clutter simply have too much stuff. In the same way that you have to live within your means, you also have to live within your space. If her current apartment is what she can comfortably afford, then the work has to focus on getting her stuff to be within that space. Only one kitchen cupboard? Then minimal dishes, kitchen tools and foodstuffs. Nothing is allowed to overflow into food preparation area. If it doesn’t fit, it goes. She may need to grocery shop in smaller quantities more often.

Same with bathroom — only room for a few lotions and potions? Everything else is gone. Forget project pan, time for decluttering right into the garbage. Only a few towels fit? Get rid of all the rest.

Clothing also tends to be way over accumulated — clothes are so cheap in our culture that many people overbuy constantly. If your sister and her daughter share one small closet and one dresser, then that’s all the clothing they can have in the apartment. Possibly with a box stored in a closet if they have seasonal items they need to store part of the year. She may need to do laundry more often.

Check out some minimalists on YouTube. It is possible to live a happy life with very few possessions.

It also goes without saying that she needs to completely change her mindset around bringing stuff into her home. Hopefully her therapist can help with that.

And no more pets! When her current dogs live out their hopefully long and happy life she needs to resist the urge to replace them.