Feeding: Every 5 days sounds like a good feeding schedule. I've left mine for up to a week with no ill effects. Just make sure your sister isn't going to feel sorry for them and give them a little extra as it's been so long since thier last meal. If you don't trust her, pre-measure the food and have her put in that amount.
: I'd turn it down or off, unless you're using it to regulate your pH. The plants won't grow as fast without it, but they won't suffer otherwise. Unless you're generating your CO2
, you might want to reduce the turbulance so you don't have to buy as much of it. (Long run project, not for your holiday.)
Water Change: I'd actually skip the big water change. You want everything to be as stable as possible, not diffrent from usual. Do your regular maintenance before you leave, and again when you get back.