There’s no one-size-fits-all here. It’s really tough when you are dealing with kids that are not your own, because you can’t dig into the toolbox that you have acquired. You don’t know what works, and every minute is an experiment.
The only thing I can suggest is to not micro-manage their interpersonal relationships with each other. You’re bound to immediately find yourself in a situation where you are perceived as siding with someone. Try to be an impartial party who gives them the opportunity to work things out. You can try even giving them more agency than they might be used to. Kids can surprise you when they are given responsibility.
So, for starters, make sure their basics are taken care of: food, water, comfort. Depending on the ages and temperaments of these kids, avoiding hunger is the first proactive step in minimizing chances of things going south.
And when things start “going south” with these kids, you can always try a redirect. Sometimes boredom can trigger a bad scene. Move onto something else – but let them choose. Offer them a couple of choices and see if they can work it out for themselves. This will require some negotiation between them, which is good. But rather than unproductive arguing, this type of negotiation has immediate results. If they can work together to come up with a choice of activity, this could very well carry over into their interactions while with you overall.
If the redirect(s) are not working as planned, you can try to shake them out of their normal experience. Is it 11:00pm at night and they can’t seem to settle down and sleep? Are they fighting and driving you and each other crazy? Well, maybe you ask them if they would like to either come downstairs and help you clean the kitchen…or…would they like to go outside and look for constellations. If they bite at the latter, then set the rules. They need to follow your directions and when it’s time to go inside, they need to be quiet and go to bed. If they are young and not used to being dragged outside in their pjs to look at the stars (or whatever), then they might be quite excited about the experience and will be grateful that they’re not cleaning the kitchen. (Note: If they agree to clean the kitchen, really make them clean the kitchen.)
Crap, I’m rambling. This is really tough without knowing the kids or their ages, or having met them. Go with your instinct.