Displaying FontFamily in Combobox

My goal is to manipulate the text-styles of my application via DependencyProperties. I got a diagram in which the texts are to be manipulated in size, fontfamily, color, etc. So I'd like to use an interface similar to a rich text editor like Word.

I'm using this code in my TextStyleVM http://shevaspace.blogspot.com/2006/12/i-have-some-fun-with-formattedtext_14.html

So I have a FontFamilyProperty and a Getter and Setter for it:

        public static DependencyProperty FontFamilyProperty =
            DependencyProperty.Register(
                "FontFamily",
                typeof(FontFamily),
                typeof(OutlinedText),
                new FrameworkPropertyMetadata(
                   SystemFonts.MessageFontFamily,
                   FrameworkPropertyMetadataOptions.AffectsRender |
                   FrameworkPropertyMetadataOptions.AffectsMeasure),
                      new ValidateValueCallback(IsValidFontFamily)); 

  public FontFamily FontFamily
    {
        get { return (FontFamily)base.GetValue(FontFamilyProperty); }
        set { base.SetValue(FontFamilyProperty, value); }
    }

Then there is a ToStyle method, which sets the style for the labels of the diagram, which are to be manipulated:

        Style style = new Style();
        Binding fontFamilyBinding = new Binding("FontFamily");
        fontFamilyBinding.Source = this;
        Setter fontFamilySetter = new Setter();
        fontFamilySetter.Property = TextBlock.FontFamilyProperty;
        fontFamilySetter.Value = fontFamilyBinding;
        style.Setters.Add(fontFamilySetter);

        return style;

Now this works for a TextBox. The textbox displays the current FontFamily, and if I enter a new, valid FontFamily like Arial into the textbox the FontFamily of the labels are changed.

However,