Hypothetical situation, let's say we have a great familly meeting, where everyone have the same surname but some members of the familly see each other for the first time. Of course, introductions are made, so everyone knows each other names. Here goes my question: How would one person address the other when they barely know each other? I mean, they can use the surname, but that would be confusing, since everyone has the same. Will they make an exception and use their names instead, even though they see other members of the familly for the first time?
I'd agree with the firstname-san in direct-greeting and 3rd-party mentions , although once conversation is going i'd reckon any usual kochira-sochira patterns and (not-even-necessarily) relational titles (ojisan/obasan, uncle/auntie etc, possibly dropping the o and even perhaps the san later if the folk are friendly and the meeting/circumstances casual) for older folk are going to seep in.. But definitely firstname-san(/-sama for a higher formality; clan heads/relations//functions etc?) would work generally, i believe.
((I've seen colleagues with the same surname move easily enough into firstname-san patterns, and while these folk in the question might not really know each other they .are. still family at the end of the day..? ..is what i'm thinking. n_n"; ))
How do you address your own father? Do you use his last name or first name?
Most people use neither. Most people will say "Dad".
That is the same with all other family members.
Most people will address one another using "uncle", "aunt", "cousin", etc.
Usually the first name is used only to address children.