It may depend on what your aunt will let you do, as well as your willingness to do the heavy lifting. This is not the best time for this project. Her peonies are probably in bloom. Maybe you are going to visit in the fall. In the fall I would dig the whole plant up using care to not break off any more of the tuberous roots in the digging. With a strong stream of water wash off all the soil. In the fall you will be able to see the buds for next years growth and with a sharp knife cut up the roots leaving 3 to 5 five buds on each piece. There will be some tuberous roots where no buds are visible. Plant them in pots anyway because the energy stored in the fleshy roots will often cause advantageous or dormant buds to grow. From one old peony I can usually get up to 10 viable plants. If you are visiting your aunt now I would be content to cut a division from the plant in the ground from the edge of the root mass and not disturb the rest of the plant. Al