First of all how many of each fish are in each tank? And how big are the tanks? This will determine whether or not water changes will be necessary for the second 2 weeks and how bad it would be for them to be overfed.
Does your brother know how to look after fish? If so you can ask him to do a water change at the end of his 2 weeks so the house sitter doesn't have to do water changes/can do less water changes.
I would ask your brother to feed the fish once every 2 days and make sure he only feeds a very tiny amount (less than you normally feed). I have found family and friends always overfeed fish while I'm away. Feeding even less will reduce waste being created, make the fish more likely to eat it all (so less uneaten food) and gives more room for error.
Try to do a large water change (about 80%) or two just before you leave and as soon as you get back.
Write down notes and stick them to the tanks. If the brother and your house sitter are not fish people try to keep the notes straightforward and don't use any fancy words, like "nitrogen cycle".
Edit: I also label all my fish food pots with numbers which correspond to numbers on the written instructions and numbers on the tank so whoever is looking after the fish knows which tank gets fed what.
I put food in small containers(powdered food) and sandwich bags with feeding instructions on sticky notes attached to the tanks. Feed the fish 3 times a week to cut down on the waste produced by them. Things like Hornwort and Duckweed can help deal with nitrate buildup, but the duckweed is a pain to clean up later.