Someone else not handling Matthew's death very well is his mother, Cousin Isobel. She heartbreakingly says, "When your only child dies, then you're not a mother anymore. You're not anything, really." *sniffle*
“Mary and Tom Branson and Isobel Crawley play the scene together and it’s with the children, all three of them are widows. … They all reflect on their great, deep loves. … It’s a beautiful scene where they describe how deeply they loved their partners and before it gets maudlin, Isobel says, ‘Aren’t we the lucky ones?’ and talk about just having known these deep loves.”
From: http://blog.zap2it.com/frominsidethebox/2013/08/downton-abbey-season-4-does-thomas-tell-lady-cora-about-the-soap-incident.html