add element siblings

backend/app/Element/UseCases/GetElement/GetElement.php

@@ -27,10 +27,19 @@ class GetElement
             throw new NotFoundException('Element not found');
         }
 
+        $parentElement = $element->getParentElement();
+        if ($parentElement === null) {
+            $siblingElements = [$element];
+        } else {
+            $siblingElements = $this->elementRepository
+                ->findByParentElement($parentElement);
+        }
+
         return new GetElementResult(
             element: $element,
             childElements: $this->elementRepository
                 ->findByParentElement($element),
+            siblingElements: $siblingElements,
         );
     }
 }

backend/app/Element/UseCases/GetElement/GetElementResult.php

@@ -8,10 +8,12 @@ class GetElementResult
 {
     /**
      * @param Element[] $childElements
+     * @param Element[] $siblingElements
      */
     public function __construct(
         private Element $element,
         private array $childElements,
+        private array $siblingElements,
     ) {
     }
 
@@ -27,4 +29,12 @@ class GetElementResult
     {
         return $this->childElements;
     }
+
+    /**
+     * @return Element[]
+     */
+    public function getSiblingElements(): array
+    {
+        return $this->siblingElements;
+    }
 }