I think this may be better. I'm going to try to utilize this in my Super Compatch to see if it works until you find a solution more to your design. Great Mod BTW!

house_vendetta.0001 = {
type = character_event
hidden = yes
#declare_house_rival = {
trigger = {
has_trait = vengeful
NOT = { has_character_modifier = blacksheep }
}

immediate = {
house = {
every_in_list = {
variable = killed_house_members
limit = {
killer = {
is_alive = yes
OR = {
highest_held_title_tier >= tier_duchy
primary_title = { is_mercenary_company = no }
}
}
killer = {
NOR = {
any_relation = {
has_relation_flag = {
target = ROOT
relation = rival
}
}
any_relation = {
has_relation_flag = {
target = ROOT
relation = nemesis
}
}
} # OR
}
} # Limit
killer = { set_relation_rival = ROOT }
} # Variable List
} # House scope where the list is stored
} # Effect
}