Maybe it's just where I am in the process, but it seems more nauseating and emotionally manipulative to me than touching. Why would you thank someone for creating a wonderful family...which you're actively working to break apart?
It's possible it's a manipulation attempt. It's also possible she feels gratitude towards the father of her children for helping to create and raise them, and getting an "XH" card was an attempt to avoid any misinterpretation.
I'll give her the benefit of the doubt and say it's not conscious. But based on her personality, it could be a way of pacifying after realizing I'm standing up. She will vacillate from rage (potentially narcissistic in nature), to calming and friendly. Which I guess is manipulation, conscious or not doesn't make a difference.
