Just as there are different kinds of relationships you might have with your relatives, there also are different levels of relationships that sister bird clubs can have.  

The first level is to find out about potential sister clubs.  
The second level is to establish a relationship with one or more sister clubs
The third level is deepen an already existing relationship with a sister club.

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These are not stages that clubs have to go through.  Having different levels simply acknowledges that some users of this website might want only a minimal relationship with other clubs while other users might want to have deep interactions with their sister clubs.

And, yes, clubs are encouraged to have relationships with more than one sister club.  These relationships might be at different levels.  There are no rules about that.  It is up to individual clubs to decide what is best for them, and what kind of relationship they want to nurture with other clubs.