Only one other poster (male) mentioned that he wrote his own thank you notes to his family. I have made that a rule in my house. My dh writes the cards to his family and friends, not me. If it’s something important, I’ll buy the card and put it in front of him, but he’ll write it. For our wedding thank yous, I gave him a list of who to write to, and made it a joint evening (or 2 or 3) of writing them. His family doesn’t get a lot of birthday cards, and the cards they get are not nearly as poetic as the ones my family gets. But if they’re offended, they know who to blame. Harsh, maybe, but a way to keep the sanity for me. I try very hard not to offend people and obsess over it if I think I’ve offended. Hopefully, the DIL here is not so sensitive.
To the OP, I would take issue only with your son, who shouldn’t refer to you by your first name. I don’t think it’s fair to critique your DIL for starting the letter with your first name (even though she knew he’s sign it). I’d be more likely to complain to him that he can’t write his own thank you card to his own mother. BUT, I wouldn’t make an issue of this to him (and definitely not to your DIL), because it is an isolated incident. If you commonly get things addressed and signed to you this way and you really can’t just let this issue be over the years, you might eventually say to your son (in a casual way): “Hey what’s the deal with you calling me by my first name, eh?” or “Why are you making your poor wife write all the notes to me? You realize you’re signing it, as though you called me (name)?” But I’d be really careful about doing that (especially anytime soon) because it comes close to a criticism of your DIL, which you should save for something really important.
I sympathize with your emotional reaction to this - it’s probably a double whammy, feeling a loss because your son is now a married man and even less your little boy than before (I don’t look forward to that day myself), and at the same time, feeling like he is losing respect for you and calling you your first name. I would be shocked though, if what you perceived as disrespectful was at all intentional on the part of the newlyweds.
God Bless,
TKC