As was already mentioned SunExpress is a 50:50 daughter of TK+LH.
What has not yet been mentioned is that Pegasus or their holding company are the second biggest shareholder in Air Berlin (now oneworld-member and Etihad-daughter). Pegasus and Air Berlin together own and run Air Berlin Turkey and some Pegasus flights are available as AB-codeshares. Although I doubt this will entitle oneworld frequent flyers to status points, this may be a possibility to earn bonus points.