The FadeOtherSeries hover mode doesn't work when the lightweight render options (Direct2D and Bitmap) are used.
You can find one way to work this around in the attached project.
Added support for FadeOtherSeries HoverMode when working with Direct2D and Bitmap render options through the RadLegend by hovering RadLegendItems. With the faster render options there is a limitation that there is no hit-testing support from WPF hence the FadeOtherSeries HoverMode will not be supported when hovering a series.