Change the Visibility
binding to:
<GroupBox Grid.Row="7" Grid.Column="1"
Visibility="{Binding
RelativeSource={RelativeSource FindAncestor,
AncestorType={x:Type UserControl}}
Path=DataContext.IsMonthlyInterestChecked,
UpdateSourceTrigger=PropertyChanged,
Converter={StaticResource VisibilityConverter}}" >
Then add a property IsMonthlyInterestChecked
to your ViewModel
and bind RadioButton
to it:
<RadioButton Grid.Column="0" x:Name="rdbMonthlyInterest"
Content="Monthly Interest"
Style="{StaticResource RadioButtonWhite}"
IsChecked="{Binding IsMonthlyInterestChecked}"/>