Well, these tips could be used for any Pokemon game, but for the sake of this question it'll only apply to Pokemon Moon.
Have a trained team. Now don't me wrong, your sister's team doesn't need a full level 100 team, but having your team around the same as the Pokemon in the trials or the kahuna if not a few levels above that in order to give yourself a slight edge. Higher level = better stats.
Have a team that covers lots of types You don't necessarily need to have a full team of 6 Pokemon by the end of the second island, but it's a good idea to have a team that will be super effective against asmany different types as possible.
Prepare for the next trial/Kahuna For this one you will need to know the type of the next trial/Kahuna you'll face, but if you do it would be a good idea to have at least one Pokemon that is super effective against the next trial/Kahuna to make it easier to beat. This is also where the last point comes in as the more diverse the team, the better odds you have.
These are the main tips I can give so I hope they help your sister get to the third island and eventually beat the elite four.
