You're forget to analyze HitTest of received HitTestInfo, see the correct code below:
Private Sub grdSiteList_MouseDoubleClick(ByVal sender As Object, ByVal e As System.Windows.Forms.MouseEventArgs) Handles grdSiteList.MouseDoubleClick
Dim grid As FlyGrid = CType(sender, FlyGrid)
Dim myPoint As Point = New Point(e.X, e.Y)
Dim ht As HitTestInfo = grid.GetHitTestInfoAt(myPoint)
If (ht.HitTest And HitTest.OnNode) 0 Then '<---determine where is the mouse cursor
Dim node As NodeBase = ht.port.Rows.GetNodeFromRow(ht.Row)
If Not IsNothing(node) Then
' Put your Double-click code here...
End If
End If
End Sub