I would like to mount a sinatra application in my rails app. But I would like this one to share the same layout.
The iframe could work but do you have any other idea
I think that using the append_view_path in your rails application will work a little bit better. Just append the Sinatra views to your Rails app and it will look there after looking in app/views.
The Crafting Rails Applications book by José Valim has a lot of documentation on that topic (rendering views from other sources), you may want to look at that.
Also, this Railscasts can help: http://railscasts.com/episodes/222-rack-in-rails-3