All the mobile Ryzen generations have been great at 15 W, which are models ending with -U for both AMD and Intel, and what you most likely are looking for unless your sister needs beefy graphics.
The CPU model is usually the least to worry about, it's all about implementation. Watch out for models with crappy displays or subpar batteries.
I have a 2700U (8 threads) laptop, I bought it second hand for $200 two years ago and I'm very happy with it. Battery life is above 5 hours, and this is no expensive model to begin with. It DOES NOT run hot.
What surprised me the most is that it's runs Metroid Dread (2021) in Switch emulator at 35+ FPS, I expected more like 5 FPS.