Viking River Cruise for those with walking difficulties

Hi, if anyone is considering a Viking River cruise, I just posted a review on the River Cruising Forum. I travelled with my sister, who has MS and back issues. She uses a cane to walk. She can do steps, but walks slowly and can't walk far without resting. A river cruise is very doable for someone with these limitations. But, it would be difficult for someone in a wheelchair or walker - sometimes the ships have to raft together and you need to go up stairs to the upper deck to get to shore. I also described Paris (our tour started from here) for those with disabilities. I'll try to address any specific questions if anyone has any after reading the review.