My ex had a new baby 4 years ago, when my DC were 11 and 8 - and I felt very similar to you when I found out. Really upset, even though I knew I didn't really have any right to be. I was worried they'd be pushed out of their dad's life, that money and space in the house would be more stretched, etc. And just kind of hurt by it all.
But tbh it hasn't been that bad overall - my DC are very fond of their little brother. I think it was me who found it hard emotionally, rather than them. Their dad's energies are spread a bit more thin I guess, but with quite a large age gap there's not really the jelousy you get with young children who get a new sibling - I don't think they'd say that overall it was a bad thing. Their DSM is as disinterested in them as ever, so no real change either way from her.
Your ex can, legally, reduce the maintenance he pays you by a small amount on the grounds of having an extra child, but mine didn't bother (or didn't dare, maybe). It's also possilbe from what you say, that they won't stay together, which could be hard in a different way I guess. Complicated for your ex to be jiggling different children coming at diffferent times, but that's all quite a way off.
Can you tell him you've heard and ask when he plans to tell the kids? Or leave it til she's showing and then ask? But I guess if it's early on, unplanned and he's not happy about it have they actually ruled out abortion yet? So I definitely wouldn't tell your kids yourself until it's much further on if he hasn't done.
I wouldn't be too judgmental about them being irresponsible - it is possible to have an unplanned pregnancy even when you have taken all sensible precaustions.
At least your ex has enough space he can accommodate your DC and the new one if he wants - mine were sharing a one bed flat with their dad, his DW and the new baby for two years. They now have a two bed, but it's still crammed.