DH decided to write a vague thank you card and focus more on hoping his cousins are well etc. Thanks to everyone for the suggestions!
I was about to post a rather impish solution - and I still can't resist!.
Since DH is the one who opened the gift and they are his relatives and since you were going to write the thank you notes (as I understood from your 1st post), DH could have called the relatives "on the sly" and "fessed up" while asking for their secret help. Eg:
DH: "Aunt and Uncle, I messed up big time and need to beg for your help so I don't get in the dog house with DW. I'm already in trouble for opening presents without her being there, but I made it worse by not remembering who gave us what!!! DW is getting ready to write the thank you cards and if I can't tell her who gave us what (since DH would present the story as their present wasn't' the only one he forgot, they won't feel singled out), I'm sleeping outside!
Please forgive me for not remembering, but could you please tell me what you gave us and not tell DW what an idiot I am?? "
Gets you off the hook and humorously puts him on it.
But a general thank you note from him is best to avoid any backfiring.