I'm a recovering hoarder. The bad news is that you cannot change a hoarder's behavior until the hoarder wants to change. Also, by the time a person is at their "wits end" they are usually not emotionally and rationally in a place where they can really help the hoarder. Because things that are the opposite of helpful: yelling, threatening, shaming, taking matters into your own hands... Are the natural response to the feelings of anger and fristration overwhelming the non-hoarder.
What the hoarder needs are huge helpings of patience, understanding, and encouragement. That is one thing that is helpful about an online community like this, there is often someone who can sincerely praise you for throwing out a pizza box. Which genuinely helps.
I think in your place, I would focus on the home I was living in. Does your sister ever come over? What is your mother hoarding? How hoarded is the space (can you see the floor? How much floor? Is it possible to safely cook?)
Is the hoard growing, and if so,what is the source of the addition material - how is it coming into the house?
Understanding why your mom hoards is an important first step because different motivations for hoarding can have different solutions. Is it possible for you to have a non-judgemental conversation with her about why she is keeping things? Do you think she would be able to be honest with you? It can't be a confrontational "Why are you keeping this (subtext "garbage")?!" It needs to be a genuine question. "So, can you help me understand why you are keeping these magazines? What is it about them that is important to you?" (Subtext, I'm willing accept your values as valid - you don't have to buy into them, just understand what her values are.)