That's terrible OP - she's obviously counting on your continued good nature and sense of responsibility (as in, doing the decent thing, NOT that you have any legal responsibility) to ensure she still gets "her" time away from her own children.
Clearly, her dumping them on you - and what else is it if not dumping seeing as the ignorant cow literally ran off before seeing or speaking to you ? - is all about how it benefits HER, and nothing to do with her wanting to continue the sibling relationship. If that was her aim she'd speak to you like an adult to come to a mutually beneficial arrangement so the children - which is something you've already more than demonstrated you'd be amenable to.
But obviously, you can't be expected to maintain the previous arrangement just to suit her. I understand why you didn't want to drive them back and expose them perhaps to arguing and/or the no-show of their mum, and/or to make them feel "rejected" by you - but this can't be allowed to continue. What you actually do about it is another thing and I don't know enough to offer any real advise ..... the things which immediately spring to mind are to either call non-emergency police or social services, but appreciate that could be traumatic for the girls. But yes, do seek legal advice for the future - it sounds even as if you may need to take an injunction of some description out against her to prevent her doing this. That would NOT mean you were relinquishing all desire to see them again - or that you want to cut the siblings' ties and feelings for each other (before you start to feel "guilty") - but that such contact must only ever be arranged when ALL adult parties concerned have agreed.
And yes - she is an appalling excuse for a mother. As I said before, however lovely the stepmother is, the instinctive reaction of most mothers in a situation like this would be to step up and take on more of their own children's care - regardless of previous arrangements. Most mothers would want to spend more time with their own children to help them through such a traumatic time - and most decent people would recognise and be grateful for any support and care a stepmother would be able to offer, and NOT take the piss about it with irresponsible dumping and no regard towards the cost to the stepmother of all this (however much you care for them, kids still cost obviously!)
OP - don't let her bamboozle you over this. What DontmindifIdo said is almost certainly true - when she realises she could potentially lose ALL her free time, she's likely to back down.
I'm so so sorry you're having to go through all this stress and extra work. What she's doing is pretty monstrous - both to her girls and to you. She has no compassion for them or you over your husband's death - she's just appallingly selfish and it almost beggars belief. What if one of them had an accident or fell seriously ill and you couldn't contact her ?