We've talked about this before.
Many years ago, Yamaha loaned Korg (the families are very friendly in Japan) some money for their company. The loan was paid off and there is no truth to Yamaha owning part of Korg. However, Korg does buy some of Yamaha's key beds to put in some of their models. That's it!
George Kaye
Kaye's Music Scene
Reseda, California