If you want to customize your JSON rendering response and want complete customized and controlled output with your own logic behind. Here is the solution for you.

ItemSiblingRenderingContentsResolver.cs

using Microsoft.Extensions.DependencyInjection;
using Newtonsoft.Json.Linq;
using Sitecore.Data.Items;
using Sitecore.JavaScriptServices.GraphQL.LayoutService;
using Sitecore.LayoutService.Configuration;
using Sitecore.LayoutService.ItemRendering.ContentsResolvers;
using Sitecore.Mvc.Presentation;
using System;


namespace MyCore.Foundation.LayoutService.RenderingContentsResolvers
{
  /// <summary>
  /// Render current item siblings that belong to the same parent
  /// </summary>
  public class ItemSiblingRenderingContentsResolver : RenderingContentsResolver
  {


    private readonly IServiceProvider serviceProvider;


    public ItemSiblingRenderingContentsResolver(IServiceProvider serviceProvider)
    {
      this.serviceProvider = serviceProvider;
    }


    public override object ResolveContents(Rendering rendering, IRenderingConfiguration renderingConfig)
    {
      if (rendering.Item.Parent.TemplateID == IDealConstants.TemplateId)
      {
        var contents = (JObject)base.ResolveContents(rendering, renderingConfig);


        var query = rendering.RenderingItem.InnerItem[Sitecore.JavaScriptServices.Core.FieldIDs.JsonRendering.GraphQLQuery];
        if (string.IsNullOrWhiteSpace(query))
        {
          return contents;
        }


        var graphqlResolver = ActivatorUtilities.GetServiceOrCreateInstance(serviceProvider, typeof(GraphQLAwareRenderingContentsResolver)) as IRenderingContentsResolver;
        var graphqlContent = (JObject)graphqlResolver.ResolveContents(rendering, renderingConfig);


        ////if both rendering content + graphql content need to be rendered uncomment below lines.
        //contents["additionalData"] = graphqlContent;
        //return contents;
        return graphqlContent;
      }
      return null;
    }


    protected override JObject ProcessItem(Item item, Rendering rendering, IRenderingConfiguration renderingConfig)
    {
      JObject jRootObject = new JObject();
      if (item.Parent.TemplateID == IDealConstants.TemplateId)
      {
        JArray jarrayObj = new JArray();
        foreach (Item obj in item.Parent.Children)
        {
          if (item.ID != obj.ID)
          {
            //JObject jobject1 = this.ProcessItem(obj, rendering, renderingConfig);
            JObject childObject = new JObject()
            {
              ["id"] = (JToken)obj.ID.Guid.ToString(),
              ["name"] = (JToken)obj.Name,
              ["displayName"] = (JToken)obj.DisplayName,
              ["ShortTitle"] = (JToken)obj.Fields["ShortTitle"].Value,
              ["LongTitle"] = (JToken)obj.Fields["LongTitle"].Value,
            };
            jarrayObj.Add(childObject);
          }
        }


        jRootObject.Add("Siblings", jarrayObj);
        return jRootObject;
      }
      return null;
    }
  }
}