Difference between "bisnipoti" and "pronipoti"?

I see bisnipote attested from the 14th century and bisnonno from the 17th, which I find quite interesting and curious; pronipote is attested from the 15th century. (Source: Sabatini-Coletti.)

A bisnipote is usually a grandson's or granddaughter's child (in direct descent), whereas pronipote can also be used in extended meaning as a descendant of a collateral line. Somebody can be pronipote of a prozio: at some point the two have an ancestor in common with more than three steps. Two steps would make the two people cugini, three steps zio/zia and nipote. On the other hand, it's common in Italy to call cugini two people having in common a grandmother/grandfather.

Prozio/prozia is, according to the same dictionary, a mother's or father's uncle/aunt (17th century), but it's also used with more generations in between. However, if I had to address my mother's uncle, I'd simply use zio: prozio and, similarly, pronipote are rather formal and used for speaking of the person, not to the person.

Note that in Italian there is no distinction between granddaughter/grandson and nephew/niece: both are nipote.