With my initial proactive communication, I sent him an email. All of this was told to me verbally when he dropped the girls off last night. Yes, he will do that on purpose so nothing is "in writing". That is so when I come back with another email he will claim he "never said anything like that"...
So on feelings alone, I predict if I email him like LdiJ said, he will tell me that he never said he was going to leave the girls alone all weekend and that his plans changed, or now he was going to take them with him, or some other excuse.
Basically I will be left in the dark as to where the girls will be.
This is one of the very few cases that I would deny visitation. Make sure everything is covered in writing and offer for the 15 yo stepsister to stay with you so she can see the kids, but I wouldn't allow it either. You might get your hand slapped by the court, but even that is unlikely- as long as you can document your objection to the kids staying with a 15 year old over the weekend ON SAFETY GROUNDS (not simply because you don't like her) and you offered makeup time.
I RARELY agree with withholding visitation, but there are some times when it's just unavoidable.
Of course, as soon as you tell him that, he may say "I changed my mind, I'm staying home that weekend" then pick the kids up and leave them with the older child, anyway. In that case, you'd have to find some way to pick them up (and they would have to have some way to notify you).
As I've stated before, he likes to push buttons and this is how he does it.
I really hate hearing that phrase. In almost every situation I've ever seen, it takes two to tango. Even if he's the primary instigator, your response is part of the problem, as well. And I doubt very much that you're perfect and never "pushed his buttons".
At some point, it's time to let the old marriage go and get on with your life. When you do that, he won't be able to push your buttons any more - and everyone will be a lot happier.
There is no stepmother, daughter is from a previous marriage. And dad won't let her visit with me (although I did get visitation awarded in our decree but it was worded "at stepchild's discretion").
As for transportation, if I had a true emergency, I would find a way to get to them.
As was explained, that is a strange order. You have no legal right to visitation with the stepdaughter. At the same time, Dad is free to let the stepdaughter visit you on his time whenever he wishes, so it's also unnecessary. If Dad wants to let her visit you on his time, he can do so. Of course, if it's the same thing - where Dad really won't spend any time with the stepdaughter and is simply getting her as a babysitter, her mother is free to object.