I can't remember such a situation in my own life (I've been blessed not to have faced a terrible fate), but even if Lydia laughed out of relief, would the family have laughed too? They would have appreciated how Lydia has finally realized what her condition could have been (widowed or married to a disabled soldier) and they might have commented to that effect, But laugh...? I can't quite see that from the well-brought up ladies such as you have described in your story.
