house_relation_is_valid_to_start_trigger

house_relation_is_valid_to_start_trigger = {
	$HOUSE$ ?= { save_temporary_scope_as = house_1_temp }
	$OTHER_HOUSE$ ?= { save_temporary_scope_as = house_2_temp }
	trigger_if = {
		limit = {
			exists = scope:house_1_temp
			exists = scope:house_2_temp
		}
		scope:house_1_temp = {
			NOT = { this = scope:house_2_temp }
			# IF HOUSE RELATION DOES NOT ALREADY EXIST, ENSURE IT IS WORTHWHILE TO START
			trigger_if = {
				limit = {
					NOT = { has_house_relation_with = scope:house_2_temp }
				}
				# MINIMUM START CHECKS
				house_head ?= { is_playable_character = yes }
				scope:house_2_temp.house_head ?= { is_playable_character = yes }
				# EXTRA CHECKS FOR AI
				trigger_if = {
					limit = {
						NOR = {
							any_player = { house ?= scope:house_1_temp }
							any_player = { house ?= scope:house_2_temp }
						}
					}
					# DON'T CREATE LOTS OF RELATIONS FOR TINY AI HOUSES
					any_house_member = { count >= 5 }
					scope:house_2_temp = {
						any_house_member = { count >= 5 }
					}
				}
			}
			# GENERAL CHECKS
			OR = {
				# HOUSES IN SAME DYNASTY CAN ALWAYS MAKE RELATIONS
				house_dynasty = scope:house_2_temp.house_dynasty
				# TIER CHECKS IF BOTH INDEPENDENT
				trigger_if = {
					limit = {
						house_head = { is_independent_ruler = yes }
						scope:house_2_temp.house_head = { is_independent_ruler = yes }
					}
					house_head = {
						tier_difference = {
							target = scope:house_2_temp.house_head
							value < 2
						}
						tier_difference = {
							target = scope:house_2_temp.house_head
							value > -2
						}
					}
				}
				# SAME REALM CHECK IF NOT BOTH INDEPENDENT (SHARED TOP LIEGE)
				trigger_else = { house_head.top_liege = scope:house_2_temp.house_head.top_liege }
			}
			# DIPLO RANGE IS ALWAYS REQUIRED
			house_head = { in_diplomatic_range = scope:house_2_temp.house_head }
		}
	}
	# SCOPES ARE INVALID
	trigger_else = { always = no }
}

As written in game/common/scripted_triggers/10_tgp_house_relation_triggers.txt.