Say I know a woman named Satou Hanako, whom I would refer to as "Satou-san". If I meet her family (mom, dad, big brother, big sister, etc.), and they introduce themselves by name, how should I refer to each of them while conversing with them? Would I be allowed to refer to Hanako by first name? Should I just refer to the family members by their relationship (e.g. "onii-san", "okaa-san", etc.)? For the record, the context I'm thinking about is adult co-workers.
Normally your options are (1) FirstName-san or (2) otou-san, okaa-san, etc.
Personally, FirstName-san would sound too friendly assuming the parents are much senior to you. This applies to other members depending on age differences. So, using the usual relationship terms may be safer.
That said, I don't think you call them by name so often in reality anyway. At least much less frequently than in English.
It is common for the father of a girl to get annoyed if called otou-san by the boyfriend of the girl because using otou-san suggests the father and the boyfriend are family already. Sometimes the father expects to be called LastName-san.
(Despite this, using otou-san sounds most neutral to me.)