diff --git a/api/features/multivariate/serializers.py b/api/features/multivariate/serializers.py
index 5b7ade60c7a8..7142aa8f83e8 100644
--- a/api/features/multivariate/serializers.py
+++ b/api/features/multivariate/serializers.py
@@ -77,16 +77,23 @@ def validate_key(self, value: str | None) -> str | None:
             raise serializers.ValidationError(RESERVED_VARIANT_KEY_MESSAGE)
         return value
 
-    def validate(self, attrs):  # type: ignore[no-untyped-def]
+def validate(self, attrs):  # type: ignore[no-untyped-def]
         attrs = super().validate(attrs)
+        
+        feature = attrs.get("feature") or getattr(self.instance, "feature", None)
+        default_percentage_allocation = attrs.get(
+            "default_percentage_allocation",
+            getattr(self.instance, "default_percentage_allocation", 0) if self.instance else 100
+        )
+        
         total_sibling_percentage_allocation = (
-            self._get_siblings(attrs["feature"]).aggregate(
+            self._get_siblings(feature).aggregate(
                 total_percentage_allocation=Sum("default_percentage_allocation")
             )["total_percentage_allocation"]
             or 0
         )
         total_percentage_allocation = (
-            total_sibling_percentage_allocation + attrs["default_percentage_allocation"]
+            total_sibling_percentage_allocation + default_percentage_allocation
         )
 
         if total_percentage_allocation > 100:
@@ -99,16 +106,19 @@ def validate(self, attrs):  # type: ignore[no-untyped-def]
         return attrs
 
     def _validate_key_is_unique(self, attrs: dict[str, typing.Any]) -> None:
+        key = attrs.get("key") or getattr(self.instance, "key", None)
         key = attrs.get("key")
         if key is None:
             return
-        if self._get_siblings(attrs["feature"]).filter(key=key).exists():
+            
+        feature = attrs.get("feature") or getattr(self.instance, "feature", None)
+        
+        if self._get_siblings(feature).filter(key=key).exists():
             raise ValidationError(
                 {
                     "key": "Multivariate option with this key already exists for the feature."
                 }
             )
-
     def _get_siblings(self, feature: Feature):  # type: ignore[no-untyped-def]
         siblings = feature.multivariate_options.all()
         if self.instance:
diff --git a/api/tests/unit/features/multivariate/test_unit_multivariate_views.py b/api/tests/unit/features/multivariate/test_unit_multivariate_views.py
index 3fb923d257b3..42609d657421 100644
--- a/api/tests/unit/features/multivariate/test_unit_multivariate_views.py
+++ b/api/tests/unit/features/multivariate/test_unit_multivariate_views.py
@@ -105,3 +105,25 @@ def test_list_mv_options__feature_in_other_project__returns_404(
 
     # Then
     assert response.status_code == status.HTTP_404_NOT_FOUND
+
+
+def test_multivariate_feature_option_create_omitted_allocation_fails_if_siblings_exist(
+    feature,
+):
+    # Given an existing option taking up 50%
+    MultivariateFeatureOption.objects.create(
+        feature=feature,
+        default_percentage_allocation=50,
+        type="unicode",
+        string_value="control",
+    )
+
+    # When we try to create a new one without specifying an allocation
+    # (Serializer defaults to 100, so 50 + 100 = 150 > 100)
+    serializer = MultivariateFeatureOptionSerializer(
+        data={"feature": feature.id, "type": "unicode", "string_value": "variant"}
+    )
+
+    # Then it should fail validation
+    assert serializer.is_valid() is False
+    assert "default_percentage_allocation" in serializer.errors