Sisters I think given how new this all is to you and how insensitive your ex is you are dealing with it all really well. And I think people are being a bit hard in you telling you to accept that contact will happen, when you clearly aren't trying to stop it, just needing it to be done sensitively and for the children's benefit.
Yes contact is likely to be ordered should it go to court. But that won't happen right away and your ex will find that court rather than talking, mediation and putting the children's needs above his own wants will drag on and on and cost everyone their sanity as well as money. Talking works a lot better (and faster) than laying down the law and if you are coming across as reasonable his sol should advise him to take that route rather than court.
I'm not saying you are being unreasonable, just that you do now have to appear to be reasonable at all times and turn the tables so that he is the one being unreasonable. Then if it ever goes to court they'll be supporting you and telling him to be reasonable.
So ok...
The point of contact with the Non Resident Parent is for the children to spend time with that parent. This other child being there (or not) should be irrelevant really and shouldn't be the reason for contact happening this weekend at all.
If your ds only wants to go when then other boy is there and is refusing to go when he's not, then he is going to play with that child, not to spend quality time with his dad.
Also - I don't see anything wrong with you suggesting next weekend instead of this given that it is back to school and there have been big upheavels during the last few weeks. Not sure why people are telling you that's not on. I think that maybe they perceive that you are basing this on the significance of the weekend to you (obviously it's not so important to your ex - you're well rid of him clearly!). However I feel that back to school is a very good reason, especially as they've had such big changes and his sisters won't be there either.
I would suggest you are firm with your ex from the start and don't let him bully you on this.
Tell him that of course contact will go ahead and you expect him to maintain his relationship with the children and that you fully support that.
But then say that contact this weekend is not convienient to you all - him and you and the children (he can't accomodate them all and you had other plans already given it is a significant date for you, which you could change but given the difficulties he has accomodating them all this weekend it seems reasonable for them to stay with you and go to him next weekend - he doesn't need to know what your plans are this weekend, and he may actually think about it a bit if you say it's significant)
You could also point out that if there is a bed for the step grandson that next weekend when he's not there he'll only be short one bed which will be easier for him to sort out - That way you are sounding helpful and considerate...
Anyway... say you believe it is in your dc's best interests to settle back into the school routine this week, stick to the plans you have already made for the weekend and then all come together. Also point out that the girls are upset about everything which is only natural. And that they will all have questions and may need reassurance from each other AND him as their dad and therefore it seems sensible for them all to come together.
You could also say that the sooner they get used to coming together and not opting out or being left out for whatever reason (beds is not major, there are alternatives such as camp beds or blow up mattresses - and he'll only be one bed short next weekend) the better.
Suggest how contact will proceed as you see it (every other weekend, friday school pick up to monday school drop off) and mention that if they come the following weekend it will be 2 weeks till the step grandson is there again which will tie in nicely. Mention that he will of course have to factor in time to ensure home work is done and you'll be sure to give him copies of timetables etc when they get them from school this week so he knows what equipment, lunch money etc he'll need to provide.
Then let him run around sorting out how to do school pick up and drop off and being a real parent OR let him turn around and say he can't do those so he's the one looking unreasonable as you've offered a good arrangement.
Then throw in that once the contact is working and the dc are happy you'll discuss holiday arrangements and special events like christmas, birthdays etc ONCE everything is working and the dc are happpy about it all. He might argue but as long as you are being reasonable about everything he won't have a leg to stand on.
That's how you poke him in the eye and show the court (if it comes to it) that you have bent over backwards, are putting your children first, and that he is the unreasonable one.
I would suggets you write it out clearly, either in a letter or email and send it to him and copy it to his solicitor. Make sure it is impartial, has nothing emotional from your point of view in it and is completely child focused. If you want someone to read it through I'd be happy to give you some feedback before you send it.
Make sure you get the sols details if you don't have them already. It will cost him £50 each time the sol sends the letter to him, even when you've already copied it to him. If he wants to pay for a sol then let him pay. Him telling you about his sol is clearly meant to scare you into doing what he wants. Don't stand for it.
He will soon find out that talking is better for everyone than threatening you with sols and court. Well maybe he won't based on what you've said before...
Lastly I would add that I say all this as a person who is very much in favour of both parents having significant contact. So as long as you are being reasonable, allowing contact and putting the children first then your ex won't have a leg to stand on.
Stay strong, your children need you more than ever now, but they also need their dad. And please plan something for next weekend. Your ex can object as much as he likes but no court will order contact for next weekend even if he does try that route. By the time it gets to court the weekend will be long past. So plan something and arrange something for yourself the following weekend when they will all be at their dads.
Best wishes and big hugs. I think you're being very reasonable given how new this all is to you :(