It’s very common for elderly people to hoard. This generation grew up in the depression and struggled to make ends meet, so they saved everything. Also, the elderly start to lose control of their life, and hanging on to their possessions is one of the last things they have control over. Keep that in mind when you start to help her. Try to start by organizing things and putting them in boxes and containers that can be moved to a storage area so it is not in her way and a fall hazard. Talk to her in advance, and go slowly at her pace to see if she is willing to let go of some things.