Ok, brief back story....
My cousin is older than me by 15 years and he has a daughter "S" who is 14 years younger than me. He is my Dads sisters son. We're relatively close, not obsessively so but more so since there was a fall out between my Dad and his Mum and he no longer speaks to his Mum.
I make an effort with his son (7) and daughter (16)....keep in touch with the daughter, she was one of my bridesmaids. Always send birthday presents etc, and prior to DS (2) occasionally had the kids for cousin for the weekend so him and his wife could get away.
Monday was "S" 16th birthday, I texted early Monday morning to say HB, hope she had a good day etc....she asked would I be seeing her that night and I said I couldn't make it but would pop her card and present in the week.
Long story short due to an upset over a diagnosis for DS's speech and an HORRENDOUS argument between me and MIL (whole other story) that spanned Sunday to Wednesday - not exaggerating, was awful, I forgot. 
I have just received this text from cousin:
"Hi Soul, hope you are all ok but I'm disappointed you haven't sent S a birthday card. She looks up to you as part of her small family circle."
AIBU to think this is a tad OTT? The birthday was acknowledged and its unfortunate I forgot but the birthday present was bought just not delivered! Ready to be flamed!