Kids who don't care? Advice!

La

I have two step children and we'll there are plenty of underlying behavioural issues but I'm only wanting to talk about one. They don't care about anything when it doesn't come to themselves.. well my SO got pretty sad when he told the kids his bday was tomorrow (which happened on the 1st as his bday is on the 2nd) they looked at him and said nothing so he said again "it's my birthday tomorrow." they continued to ignore him. Well, the next morning he woke up and said to them "today is my birthday" with a big smile on his face and they didn't say anything he proceeds to say it 3 more times before the eldest said happy birthday in a low tone. This isn't the only time they don't show compassion, they honestly don't care unless it comes to something they will benefit from. They are 8 and 5 years old, I don't believe it's just them being kids as they are old enough to understand. How do we correct this behavior? I remember being as young as 4 and being so happy it was my parents bday, I would ask to make a card as well. I don't understand it, I've honestly never met kids like this before. My SO is still pretty upset on how they acted towards his birthday 😕.