You will have to open the descr_sm_factions file and change this entry in the venice section:
Code:
faction venice
culture middle_eastern
religion islam
symbol models_strat/symbol_venice.CAS
rebel_symbol models_strat/symbol_venice_rebels.CAS
primary_colour red 110, green 39, blue 24
secondary_colour red 110, green 39, blue 24
loading_logo loading_screen/symbols/symbol128_venice.tga
standard_index 4
logo_index FACTION_LOGO_VENICE
small_logo_index SMALL_FACTION_LOGO_VENICE
triumph_value 5
custom_battle_availability yes
can_sap no
prefers_naval_invasions yes
can_have_princess yes
has_family_tree teutonic yes
That will give you a regular family tree, there are several tutorials available in the text editing forum for you to add details.