Wow you are brave! I feel like I'm just getting the hang of a 4 yo. Teenagers scare me a little! But I do have some ideas, not from experience, mind you.
I think that these two are old enough to be involved in the discussion about the playstation and other ground rules - before they arrive. I think that you can say that the playstation will be limited and then ask what they think is a reasonable amount of time per day. There should be some back and forth about the time and eventually - hopefully - you should come to an agreement. then you should ask what should be the consequence of playing more than the limit or having an attitude when they realize that time is up for the day. Should the machine be taken away for 24 hours or less time allowed for the next day, etc. Essentially, figure out all the possible ramifications, get their input and agreement, and maybe even get them to sign a paper so that there is no disagreement later about what was agreed to. I would really get this done before they arrive.
Hopefully they are excited about coming to your house and will be enthusiastic about making this work. If you start to get some lip along the lines of "you didn't make us do this last year!" You can calmly say that things are different because there is another child in the house to consider and remind them that they were not always happy about the situation last time either.
About the food, I think that part of that is just the way teenagers are. They are growing and eat a lot. I think you can make it clear that these snacks are to last for a week so if they eat them all in one day there won't be any more for a week. You can also make it clear that some foods are your's or the baby's or your dh's and are out of bounds. You may want to communicate with the kids' parents to find out what and how much they eat to be sure that you have a realistic idea of the quantity necessary to keep them fed.
Also, make some options available to them for what they will do while they are there, hopefully before they arrive. The pool, museums, parks, organized sports, camp, art projects, books whatever. If they arrive with nothing to do, then the playstation will be very desirable.
I think that you should get your dh's input and agreement but then take the initiative for setting the ground rules yourself, because, as you said, you will be dealing with this all day.
Geez, I just reread your post and see that they are coming next week. Good luck! I hope you get some more ideas from posters who actually know something about teenagers!