When Melinda Nelson moved to Weiser, Idaho, in 2019, she bought a house with her mother and sister that has three steps leading to the entrance. Nelson has bad knees — “bone on bone,” from advanced arthritis, she said. But initially, the steps weren’t a problem. She could hobble up them using her cane, or else her mother, who’s 79, would help her. But Nelson’s knees got worse and worse — the result of having spent her 30s as a hotel housekeeper…