diff --git a/app/models/invitation.rb b/app/models/invitation.rb
index afafd7852..e2900c7cf 100644
--- a/app/models/invitation.rb
+++ b/app/models/invitation.rb
@@ -15,7 +15,9 @@ class Invitation < ApplicationRecord
   validates :token, presence: true, uniqueness: true
   validates_uniqueness_of :email, scope: :family_id, message: "has already been invited to this family"
   validate :inviter_is_admin
+  validate :no_other_pending_invitation, on: :create
 
+  before_validation :normalize_email
   before_validation :generate_token, on: :create
   before_create :set_expiration
 
@@ -57,6 +59,24 @@ class Invitation < ApplicationRecord
       self.expires_at = 3.days.from_now
     end
 
+    def normalize_email
+      self.email = email.to_s.strip.downcase if email.present?
+    end
+
+    def no_other_pending_invitation
+      return if email.blank?
+
+      existing = if self.class.encryption_ready?
+        self.class.pending.where(email: email).where.not(family_id: family_id).exists?
+      else
+        self.class.pending.where("LOWER(email) = ?", email.downcase).where.not(family_id: family_id).exists?
+      end
+
+      if existing
+        errors.add(:email, "already has a pending invitation from another family")
+      end
+    end
+
     def inviter_is_admin
       inviter.admin?
     end
diff --git a/test/models/invitation_test.rb b/test/models/invitation_test.rb
index 710b4447e..9895538e8 100644
--- a/test/models/invitation_test.rb
+++ b/test/models/invitation_test.rb
@@ -62,6 +62,66 @@ class InvitationTest < ActiveSupport::TestCase
     assert_not result
   end
 
+  test "cannot create invitation when email has pending invitation from another family" do
+    other_family = families(:empty)
+    other_inviter = users(:empty)
+    other_inviter.update_columns(family_id: other_family.id, role: "admin")
+
+    email = "cross-family-test@example.com"
+
+    # Create a pending invitation in the first family
+    @family.invitations.create!(email: email, role: "member", inviter: @inviter)
+
+    # Attempting to create a pending invitation in a different family should fail
+    invitation = other_family.invitations.build(email: email, role: "member", inviter: other_inviter)
+    assert_not invitation.valid?
+    assert_includes invitation.errors[:email], "already has a pending invitation from another family"
+  end
+
+  test "can create invitation when existing invitation from another family is accepted" do
+    other_family = families(:empty)
+    other_inviter = users(:empty)
+    other_inviter.update_columns(family_id: other_family.id, role: "admin")
+
+    email = "cross-family-accepted@example.com"
+
+    # Create an accepted invitation in the first family
+    accepted_invitation = @family.invitations.create!(email: email, role: "member", inviter: @inviter)
+    accepted_invitation.update!(accepted_at: Time.current)
+
+    # Should be able to create a pending invitation in a different family
+    invitation = other_family.invitations.build(email: email, role: "member", inviter: other_inviter)
+    assert invitation.valid?
+  end
+
+  test "can create invitation when existing invitation from another family is expired" do
+    other_family = families(:empty)
+    other_inviter = users(:empty)
+    other_inviter.update_columns(family_id: other_family.id, role: "admin")
+
+    email = "cross-family-expired@example.com"
+
+    # Create an expired invitation in the first family
+    expired_invitation = @family.invitations.create!(email: email, role: "member", inviter: @inviter)
+    expired_invitation.update_columns(expires_at: 1.day.ago)
+
+    # Should be able to create a pending invitation in a different family
+    invitation = other_family.invitations.build(email: email, role: "member", inviter: other_inviter)
+    assert invitation.valid?
+  end
+
+  test "can create invitation in same family (uniqueness scoped to family)" do
+    email = "same-family-test@example.com"
+
+    # Create a pending invitation in the family
+    @family.invitations.create!(email: email, role: "member", inviter: @inviter)
+
+    # Attempting to create another in the same family should fail due to the existing scope validation
+    invitation = @family.invitations.build(email: email, role: "admin", inviter: @inviter)
+    assert_not invitation.valid?
+    assert_includes invitation.errors[:email], "has already been invited to this family"
+  end
+
   test "accept_for applies guest role defaults" do
     user = users(:family_member)
     user.update!(