Is redtail Sandstorm's father or uncle?

Yes, Redtail is Sandstorm's father, and her mother is Brindleface. It is shown on Warriors Wikia that Brindleface was Sandstorm's mother, and Brindleface's mate used to be Redtail. I'm assuming Brindleface sat vigil with Redtail when he died, and they are together in StarClan. Sandstorm has a long way to go until she meets with them. But anyway, it is proven that Redtail is her father.