Bool to Visibility Converter in WPF
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}"/>