I think I know how to make both Lily and Lisa work, Lisa will make an aging 'potion' for herself so she can commandeer lab equipment without relying on Lincon (it's urgent and he's busy doing other things), however, she makes more than necessary, not paying it much mind, she drinks enough for herself and due to the urgency, she leaves immediately not bothering to clean up to seek out the necessary lab equipment once she finishes aging up (she still needs to finds clothes for herself to fit her near appearance). Afterward, Lily encounters the unsupervised aging 'potion' and drinks it mistaking it for apple juice which results in her aging up as well.
Before Lisa's return, you could have a scene with Lincon here.
After Lisa's return, and her completion of the urgent matter, she examines the effects of the aging 'potion', and turns out what Lisa believed to be temporarily turned out to be permanent. She plans to create an anti-aging 'potion' but needs time to do so. As to how long, or if you even want to undo it, is up to you.