CheckBox Checked event fires before binded collection updates

I have a custom control to show items with checkboxes inside a ComboBox. To realize this, I used a DataTemplate with a CheckBox. The ItemSource of the ComboBox uses a Binding to a ObserableCollection which contains my filtervalues. FilterValue is a cutom class implementing INotifyPropertyChanged. The properties "Content" and "IsChecked" of the CheckBox use bindings as well to use the values of my list. This control will be used in Silverlight.

Binding itself works fine, as seen here:

The problem is if I register the Checked or Unchecked event. As soon as one of the checkboxes changed it's state, the event is fired as expected, but a this moment the value in the binded list is still not changed. What i saw while debugging is that the Checked/Unchecked events are firing before the PropertyChanged event of the FilterValue. This means at the time the event is firing, I can't ask the list for all active (checked) filters. What could i do to archieve this?

FilterControl (xaml):

<UserControl
xmlns="http://ift.tt/o66D3f"
xmlns:x="http://ift.tt/mPTqtT"
xmlns:d="http://ift.tt/pHvyf2"
xmlns:mc="http://ift.tt/pzd6Lm"
xmlns:sdk="http://ift.tt/154vi9G"
xmlns:local="clr-namespace:Controls" x:Class="Controls.FilterControl"
mc:Ignorable="d"
d:DesignHeight="45" d:DesignWidth="140">

<StackPanel x:Name="LayoutRoot">
    <sdk:Label x:Name="LblFilterDescription" Content="-" />
    <ComboBox x:Name="Filter"  Width="120" ItemsSource="{Binding AvailableFilters, RelativeSource={RelativeSource FindAncestor, AncestorType=local:GridFilterControl}}">
        <ComboBox.ItemTemplate>
            <DataTemplate>
                <CheckBox Content="{Binding Path=Text}" IsChecked="{Binding Path=IsChecked, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Checked="FilterChanged" Unchecked="FilterChanged" />
            </DataTemplate>
        </ComboBox.ItemTemplate>
    </ComboBox>
</StackPanel>

FilterControl (cs):

public partial class GridFilterControl : UserControl
{
    public delegate void FilterChangedHandler(object sender);
    public event FilterChangedHandler OnFilterChanged;

    public ObservableCollection<FilterValue> AvailableFilters { get; set; }                                                                 
    public List<string> AppliedFilters
    {
        get
        {
            return new List<string>(AvailableFilters.Where(filter => filter.IsChecked).Select(filter => filter.Text));
        }  
    }

    public GridFilterControl()
    {
        InitializeComponent();

        AvailableFilters = new ObservableCollection<FilterValue>();  
    }

    public bool AddFilterValue(string filterValue)
    {
        bool found = false;

        foreach (FilterValue f in AvailableFilters)
        {
            if (f.Text == filterValue)
            {
                found = true;
                break;
            }
        }

        if (!found)
            AvailableFilters.Add(new FilterValue() { IsChecked = false, Text = filterValue });

        return found;
    }

    private void FilterChanged(object sender, RoutedEventArgs e)
    {
        //Here if I check AvailableFilters, the value is not changed yet.
        //PropertyChanged allways fires after this, what makes me unable to
        //get all currently applied filters (checked items)...
    }
}

FilterValue:

public class FilterValue : INotifyPropertyChanged
{
    private bool _IsChecked;
    private string _Text;

    public bool IsChecked
    {
        get { return _IsChecked; }
        set
        {
            _IsChecked = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("IsChecked"));
        }
    }
    public string Text
    {
        get { return _Text; }
        set
        {
            _Text = value;
            PropertyChanged?.Invoke(this, new PropertyChangedEventArgs("Text"));
        }
    }

    public event PropertyChangedEventHandler PropertyChanged;
}