Interesting... so if it's not inherited (I don't really think it is - I'm just brainstorming here), then why do so many INFJs have INFJ relatives...
Well, we generally have more than 16 relatives, and there are only 16 types. Probability says you've got a 1 in 16 chance of a person being a specific type. With more than 16 relatives there is a good chance that one of them will be the same type as you. However, due to disproportionate distribution of the types that chance is not 100%, there for it does not always happen.
1/16? Then INFJs are really 6.25% of the population.
Say it ain't so!