What do you mean by related?
Do you want to get all content that has a reference to your page?

The contentRepository has a GetReferencesToContent method you could have a look at.

You can have a look here of how to use it https://blog.i4code.nl/referencing-content-gadget/

Basically you would want to hook into the Publishevent with an InitializationModule and run your code there :)

Here's how you can hook into the event: http://world.episerver.com/blogs/Janaka-Fernando/Dates/2015/4/adding-custom-logic-to-your-publishing-step/

[InitializableModule]
[ModuleDependency(typeof(EPiServer.Web.InitializationModule))]
public class InitializationModule : IInitializableModule {
	public void Initialize(InitializationEngine context) {
		var contentEvents = ServiceLocator.Current.GetInstance<IContentEvents>();
		contentEvents.PublishingContent += contentEvents_PublishingContent;
	}

	public void Uninitialize(InitializationEngine context) {
		//Add uninitialization logic
	}

	void contentEvents_PublishingContent(object sender, EPiServer.ContentEventArgs e) {
		var contentRepository = ServiceLocator.Current.GetInstance<IContentRepository>();
		var related = contentRepository.GetReferencesToContent(e.ContentLink, includeDecendents: false);
	}
}

Yes GetReferencesToContent worked. Strange i wasn't able to find that solution by googling. I instead reflected EPiServer.UI.dll on an episerver 6 site and checked how it was done there.