I am working on Patent Ranking project where I need to consider patent citations as one of the metric. I cant find the definition of family-to-family citations. And how is it different from patent-to-patent citations.
When a patent cites a patent that's a patent-to-patent citation.
When a patent from the patent family (e.g. a divisional application or a continuation application) other than the patent itself cites a patent - that's a family citation, because the family cites the other patent.
As a side note, those citations normally include every citation made by the applicant, the examiner or a third party, so the meaningfulness differs.