They both forgot DD's 17th birthday. One forgets all the time so it's nothing new. The other forgot for the first time and did a "oh silly old me, I have the card I forgot to post it" response to my "hi, did you forget?" message .
They didn't pay her much attention when she was younger. They are a fair bit older than me and, I can only assume, were bored of that kind of parent solidarity thing. I paid loads of attention to my nieces and nephews when they were young (their kids).
They could've sent a bloody card or even just a WhatsApp message.
We generally get on pretty well as siblings. In a low level kind of way. But this is so low level it's off the bloody radar.