They look like they are old and the fabric has decayed. I've seen this happen to cotton tires many times. Can also affect rayon blend, etc. IMO they are toast and you should not ride them.
Here in lala land fabric can decay pretty quickly, I suppose from a combination of heat and ozone. Rubber goes south quickly too. I have to carefully consider sidewall fabric choices for this reason, especially for bikes that aren't ridden often.