What makes the lemon lime soda work better than the ginger ale? I always heard the sugar helps but flat pop just makes a mess. Maybe the acid in the citrus does something extra for the stems. I stick to plain water with a tiny bit of bleach to keep things clean.
Gotta jump in here because the bleach tip isn't quite right for delicate flowers... it can actually burn the stem ends and make things worse. You're better off with just a tiny drop of bleach in a big vase if anything, but skip it for gerberas. The lemon-lime soda works because the sugar feeds the flowers and the citric acid helps keep the water acidic, which slows down bacteria growth without damaging the stems like bleach can. For gerberas especially, you want the sugar to help them perk up and the acid to keep the water fresh. Flat soda does get sticky though, so I always use it fresh and just let it sit for a minute before adding the stems. Your aunt's trick sounds solid honestly, just go light on the soda so you don't end up with a sticky mess.