A couple of links for convenience.
Historical Played Bys: - A list of Medieval, Renaissance and Fantasy PBs.
ASOIAF Fancasting Resource - A
very comprehensive list of fancasting PBs for the series which can be used here if need be.
Dornish Played Bys: - a comprehensive list of Played Bys specific to Dorne
A Wiki of Ice and Fire - A comprehensive Wiki for the book series. There are spoilers here for the books, though if you are a show only viewer it is probably safe to view at this point.
These family relations were formed by way of events from the previous game:
The newest generation of
Houses Baratheon and
Targaryen are cousins by way of a Baratheon Queen and Targaryen King union
The newest generations of
Houses Tyrell.
Lannister and
Stark are cousins by way of marriage. Lady Stark and the former Lady of Lannister were both born as Tyrells.
The newest generation of
Houses Baratheon and
Tully are cousins by way of a Tully marrying the Lord of House Baratheon.
Houses Targaryen and
Martell are related as the King and his twin sister's mother was a Martell. The newest generation of both Houses would thus be cousins once removed (I think
)