Second person pronouns among family in Spain, especially with large age differences
I'm not clear about where the usted and tú divisions fall in Spanish Spanish (and I understand there's some argument as to what qualifies as castellano). Would the elderly woman be tú to everyone because they're family, or would she be usted to everyone because she's the oldest? Does tú run both ways between parents and children, or do parents remain usted even to adult children?
I'm aware of vosotros, but I've never used it outside of a classroom. I understand that it's something like "y'all," but I'm not clear as to when it would be appropriate to use it in a family group. Can you use it even if you're the youngest adult addressing a group of older ones?
Any guidance would be most welcome!