¿Cómo mostrar elementos relacionados usando DeleteView en Django?

Puede utilizar el Collector class Django uses to determine what objects to delete in the cascade. Instantiate it and then call collect on it passing the objects you intend to delete. It expects a list or queryset, so if you only have one object, just put in inside a list:

from django.db.models.deletion import Collector

collector = Collector(using='default') # or specific database
collector.collect([some_instance])
for model, instance in collector.instances_with_model():
    # do something

instances_with_model returns a generator, so you can only use it within the context of a loop. If you'd prefer an actual data structure that you can manipulate, the admin contrib package has a Collector subclase llamada NestedObjects, that works the same way, but has a nested method that returns a hierarchical list:

from django.contrib.admin.utils import NestedObjects

collector = NestedObjects(using='default') # or specific database
collector.collect([some_instance])
to_delete = collector.nested()

Updated: Since Django 1.9, django.contrib.admin.util fue renombrado a django.contrib.admin.utils

I use a cutdown modifcation of get_deleted_objects() from the admin and use it to extend my context in get_context in the delete view:

define somewhere

from django.contrib.admin.utils import NestedObjects
from django.utils.text import capfirst
from django.utils.encoding import force_text

def get_deleted_objects(objs): 
    collector = NestedObjects(using='default')
    collector.collect(objs)
    #
    def format_callback(obj):
        opts = obj._meta
        no_edit_link = '%s: %s' % (capfirst(opts.verbose_name),
                                   force_text(obj))
        return no_edit_link            
    #
    to_delete = collector.nested(format_callback)
    protected = [format_callback(obj) for obj in collector.protected]
    model_count = {model._meta.verbose_name_plural: len(objs) for model, objs in collector.model_objs.items()}
    #
    return to_delete, model_count, protected

then in your views

from somewhere import get_deleted_objects
#
class ExampleDelete(DeleteView):
    # ...
    def get_context_data(self, **kwargs):
        #
        context = super().get_context_data(**kwargs)
        #
        deletable_objects, model_count, protected = get_deleted_objects([self.object])
        #
        context['deletable_objects']=deletable_objects
        context['model_count']=dict(model_count).items()
        context['protected']=protected
        #
        return context

now you can use them in your template

<table>
  <tr>
    <th>Name</th>
    <th>Amount</th>
  </tr>
  {% for model_name, object_count in model_count %}
    <tr>
      <td>{{ model_name|capfirst }}</td>
      <td>{{ object_count }}</td>
    </tr>
  {% endfor %}
</table>
<p>
  <ul>
    {{ deletable_objects|unordered_list }}
  </ul>
</p>

Most is just copy/paste/edit/delete unwanted from django admin