Lol, no I'm not the sister!
But I do see a situation here where we are seeing only one side of the story. Yes, older kids can be put out when a new sibling is announced. They will get over it, and quite frankly at their ages they should be a bit more mature. And the OP should be encouraging them to see the positives, not feeding the negative narrative.
The relationship is 2 years old. So, it seems long term. Why would anyone question the decision to have a child, 2 years in? Just because he is younger than her?
She works hard to financially support her existing DC, and is being hanged online for not spending enough time with them. They are preteen/teen/adult. Seems you can't ever win as a single mother. Damned if you work, damned if you don't.
So what if she goes on holiday without them sometimes. Everyone needs a bloody break. I don't take my DC every time I go away. Shoot me!
I see a pregnant woman, who by the fact she is growing another human, is vulnerable. She is being vilified by her family simply for choosing to have a new relationship and a child. Surely she is entitled to happiness? I don't think that is at her other DCs expense, I just see that they are upset at a big change and acting out. A teen's tantrum is no reason to castigate a family member, and hang her out to dry. Everyone in this scenario needs to grow up.