It depends on how the parents handle it. It is normal for children to be "selfish" and not want to share. They have to learn to share first. I always made sure my two knew how to share. They are now 3 and 9, and especially the little one is great with sharing. sometimes the big one takes advantage of that (eg when it comes to candy) and then I have to intervene. They both know how to take turns, and that that's the right thing to do.

When the two you are watching are fighting over a toy, give it a couple of minutes to give them the chance to resolve it themselves. It is important for children to learn how to resolve conflict. If that doesn't work, one warning "If you don't start taking turns with the toy, I will take the toy away". If that doesn't work, the toy goes into time out. That might result in a tantrum from both at first, but eventually they'll learn.

Another way is to ask the parents how they would like you to handle those situations. Every parent has a different parenting style, and that way you know you're following their rules and not confuse the kiddos.

Disclaimer: my kids aren't perfect either, sometimes they need a reminder that they need to share and take turns. But all I have to say is "share" or "you need to take turns" and it's all good.