You could extract the common framework files into its own folder, say "common". Then, for each of the sites, you could simply require those common framework files.
For example, you could structure your common framework and site specific folders as:
- common
- common.rb
- support
- pages
- page_object.rb
- env.rb
- world_ext.rb
- site1
- features
- site1_feature.feature
- step_definitions
- site1.rb
- support
- config
- site1_domain.yaml
- pages
- site1_homepage.rb
- env.rb
- site2
- features
- site2_feature.feature
- step_definitions
- site2.rb
- support
- config
- site2_domain.yaml
- pages
- site2_homepage.rb
- env.rb
The common\common.rb would require all of the common framework files:
require 'support\pages\page_object'
require 'support\env'
require 'support\world_ext'
The site1\env.rb (and for site2) would require require the common.rb to get all of the common framework files:
require_relative '../../../common/common'