That's what I was thinking.
You, as an only child, felt like they were the nearest to siblings to you.
They had their siblings and then they had their cousin (you).
And I would have been far more likely to rush round all excited if one of my cousins had had a baby before I had mine than afterwards. Partially because a new baby felt far more exciting then, partially because having been a new mum I then knew that visiting, whereas can be lovely, also sometimes felt a bit much and also once I had my own dc it took far more to go round, and also it felt much more like an invasion when I was bringing my family than just me.
So it's not not them being mean, it just different circumstances.
And you have a choice. You can cut them off, refuse to invite them, not send photos.
But they won't mind that much, even if they notice. They've got each other, so I suspect it will be pretty much unnoticed.
So the only person you're hurting is you.
Alternatively you can put some effort into keeping the relationship alive. Phone them, tell them you would love to see them, when can they come? Or could you come to them if it's easier? The emphasis is you would like to see them not you think they ought to see your baby.
It's your relationship with them that you value, and they may develop a relationship with your baby if you keep up with them.