It is perfectly possible to be in the same clan as another player, although we are restricting this to 3 PCs per clan. These are the consequences if you do this;
Obviously you have the same clan traits as the other players in your clan. You do not get bonus traits for having more PCs in the clan.
Clan NPCs will be shared between the players in a clan, although the NPC can have a different attitude to different players (allowing them to purchase different NPC traits).
It will be much harder to keep things secret from people when they're a relative. Not that you'd hide things from your fellow clansmen, of course…
It is also possible for two different clans to be linked, whether as trading partners, bitter enemies or friendly neighbours. It is also possible for one clan to be subservient to another, although in this case the PC in the subservient clan will have to expend a large amount of effort keeping his clan happy about their station.