Look in Descr_Sm_factions:

faction sicily
culture southern_european
religion catholic
symbol models_strat/symbol_sicily.CAS
rebel_symbol models_strat/symbol_rebels.CAS
primary_colour red 70, green 70, blue 70
secondary_colour red 250, green 250, blue 255
loading_logo loading_screen/symbols/symbol128_sicily.tga
standard_index 1
logo_index FACTION_LOGO_SICILY
small_logo_index SMALL_FACTION_LOGO_SICILY
triumph_value 5
intro_movie faction/minor_intro.bik
victory_movie faction/minor_win.bik
defeat_movie faction/minor_lose.bik
death_movie faction/minor_lose.bik
custom_battle_availability yes
can_sap yes
prefers_naval_invasions yes
can_have_princess yes
has_family_tree yes

It might be worth looking into why they can sap, but it is not an option in a siege.