Thank you Attila for being my voice of reason today.
I think you are right with your FOGiness comment.
Me at the minute...
I am fearful of my parents on really base level - scared they will knock the door and enforce a confrontation - and fear of them twisting my DC's against me as they have with the rest of the family, both lately and in times past. I also feel guilty at depriving my son of what should be a nice birthday card.
Yes, I know this is not my guilt to carry, it is theirs, they have done this, not me - but I was well trained. 
The reality is the card is nasty, mean, vindictive and designed to cause nothing but angst within my chosen family. The fear I feel is again them pushing me to assume an instinctive hyper vigilant protection mode both towards myself and my children...again they taught me well.
As for obligation.......this does NOT apply.
So what have I done with the card...faff all!
I am going to sit on it and chat to DH later as he is also involved...but ultimately I will make the decision.
After I have decided what to do...then either way the card is getting shredded/burnt. I don't want to waste any more time on this than absolutely necessary...I just need to feel sure that which ever choice I make is done with clear, logical, now thinking...and not made through my foggy eyes of the past.
Thanks again Attila. x
Hmmm...
Just had hypothetical convo' with all DC's at once - bit of a family discussion - about what they would like me to do with any birthday cards if received any...they said...
They all want me to 'vet' the cards.
If the card is OK (no comments) give it to them.
If the card has comments...put it in the burning pile...hide it away so they can't see it.
but...they all want to know if they have a card.
They all also said if the card had any money in it to tell them...but to put it towards a cinema trip or takeaway for everyone to share.
Still dunno what I am going to do...but more food for thought.