It may be a wise measure. This is something which folk often think of in relation to immigration/entry requirements only, but it should also be considered in relation to any urgent medical attention which might be required. However unlikely it may be that such an incident might occur, it is best that your sister discusses with the children's parents in advance how much authority she should hold in relation to any emergency involving the kids, whether she has the right to make any urgent decisions, or whether she must consult with the parents before agreeing to any medical treatment, etc. These are sometimes delicate issues which folk either don't think about, or prefer not to discuss in advance when taking someone else's kids abroad. But in a genuine emergency, it is best that your sister is prepared and knows exactly what she should do, preferably with her status on such matters confirmed in writing so that she can provide evidence of authority if required.

David