I think it could be potentially very dangerous, but that’s just a guess. Is your sister young, and lightweight? Someone heavy walking near your spine just seems like a BAD idea. Spinal injuries should be avoided at all costs. I imagine there’s also the potential that a mis-step could crack or bruise a rib, or damage a kidney or other internal organ. Why bother with the russian roulette of it? Learn some stretches that will relieve tension in your back instead, or trade massages with your sister :) . Also, there’s a lot of people who believe than any cracking of joints is bad for you in the long run, although I’m not sure if there’s a lot of evidence to support that.