app/views/container_items/show.html.erb
<%= content_for :associated do -%>
<% if @container_item.siblings && @container_item.parent -%>
<h3> Also in container </h3>
<ul>
<% @container_item.siblings.each do |c| -%>
<%= content_tag :li, c.id -%>
<% end %>
</ul>
<% else %>
<i> This object is uncontained. </i>
<% end %>
<% end %>
<%= render('shared/data/project/show', object: @container_item) -%>