Leaving aside the fact she obviously has a history of losing her temper and screaming (lovely). Its not up to her to parent your children but their father.
Why can't he take the children on his annual leave? This is what my DH does. In fact DH has just said that your ex needs to get his priorities right. All the children are DH's, as is the case with your ex. He's never delegated parenting DSS to me, although occassionally he's been in my care when DH has an errand to do.
I suspect the reality of their relationship has not ended up as they thought. She is his affair partner, she's seeing all his warts, has a new baby and now 3 stepkids to bond with. He is also seeing what a temper she has. They've got a lot of issues to sort out. If you try and say anything, they'll just blame you for interfering and brainwashing the children.
If he is the sort of man to think childcare and domestic chores are women's work and basically dumped it all on his girlfriend, then he's got some waking up to do. I am pretty sure the girlfriend will not be letting things lie, doesn't sound like she is one to suffer in silence.
I wouldn't even go there by requesting she leave her own home? Why should she? In fact the only thing you could say if you really must is that the kids were expecting your ex to take annual leave so they could see him. But apart from that. And perhaps next time he does this. Unless they are in physical danger there is not much you can do.
From the other side of the coin, DH's ex moved her affair partner in with the kids when DH's marriage brokedown. The step father had suddenly moved in with 3 kids and took it out on the children. He frequently lost it in front of them too. DH used to get quite obsessed about checking DSS for bruises when he was little. Things seem a lot better now. Or at least DSS doesn't mention things anymore.
Of course every time DH felt he had to ask his ex why DSS was being screamed or shouted at by stepfather he just got blamed for being interfering (which he was) and the only issue was with DH. DH had some accepting to do too.