mpo_blood_brother_common_enemy_trigger

mpo_blood_brother_common_enemy_trigger = {
	scope:actor = {
		OR = {
			#You have a rival that recipient is also opposed to
			any_relation = {
				type = rival
				mpo_blood_brother_common_enemy_sub_trigger = yes
			}
			#You are at war with someone that recipient is also opposed to
			any_war_enemy = {
				NOT = { has_variable = rebel_leader_peasants }
				any_character_war = {
					is_war_leader = prev
					is_war_leader = scope:actor
				}
				mpo_blood_brother_common_enemy_sub_trigger = yes
			}
			#You are in a faction against someone that recipient is also opposed to
			liege ?= {
				any_targeting_faction = {
					any_faction_member = {
						this = scope:actor
					}
				}
				mpo_blood_brother_common_enemy_sub_trigger = yes
			}
			#You have a faction member that recipient is also opposed to
			any_targeting_faction = {
				any_faction_member = {
					mpo_blood_brother_common_enemy_sub_trigger = yes
				}
			}
			#You have a title claimant that recipient is also opposed to
			any_held_title = {
				NOT = {
					scope:recipient = {
						has_claim_on = prev
					}
				}
				any_claimant = {
					mpo_blood_brother_common_enemy_sub_trigger = yes
				}
			}
			#You are a title claimant against someone recipient is also opposed to
			any_claim = {
				exists = holder
				NOT = {
					scope:recipient = { has_claim_on = prev }
				}
				holder = {
					mpo_blood_brother_common_enemy_sub_trigger = yes
				}
			}
			#You have a house feud against someone recipient is also opposed to
			AND = {
				exists = house.house_head
				exists = scope:recipient.house
				NOT = {
					house = scope:recipient.house
				}
				house = {
					any_house_relation = {
						has_house_relation_level = feud
						any_relation_house = {
							house_head = { mpo_blood_brother_common_enemy_sub_trigger = yes }
						}
					}
				}
			}
		}
	}
}

As written in game/common/scripted_triggers/mpo_scripted_triggers.txt.