Are you married to a new one?

I pretty much exclusively buy used ~3 year old Thinkpads which meet all of your specs and then some. My last two were X1 carbons (7th gen and now a 9th gen). Each cost me about $450.

"Best overall" is nonsensical in this context. Here are some things people care about:

• Size class - a 17" laptop isn't better or worse than a 12" laptop. They're for different use cases.
• Thickness class - thicker tends to make it better at computer stuff and battery life, thinner is thinner
• Screen color quality and accuracy - important to some, not to others
• Touchscreen - yes, no, indifferent?
• Screen aspect ratio - most are 16:9, some are taller
• Screen resolution - most are at least FHD, but you can probably find something worse. Do you want better?
• Upgradeability - can you get to the SSD? Can you put a DIMM in it after you win the lottery?
• Keyboard feel and layout - some of us are more picky than others
• Power supply - does it need to run from USB-PD?
• "OS is not important", but you probably have one in mind and may need to consider compatibility
• Does GPU performance matter? It's 2026 and GPUs aren't just for gaming anymore
• Does CPU performance matter?
• How much RAM and storage is required?

I like my Thinkpad P14s III 6850u, but being a good fit for my needs doesn't mean it's the "best" or that it would be suitable for your sister.

Lenovo is usually a solid choice. My 2 in 1 cost around $600

I bought an HP (I know, I couldnt believe it either) bare bones basic laptop.

On sale for 300 US while I was there.

Removed windows, put of Fedora. Everything works, battery life is good.

Its surprising what you find on the lower end because they use well supported standard stuff to keep costs down.

No issues, been two years.

Get a Thinkpad T480, Intel ME can be disabled and it has working coreboot/libreboot builds, might someday be fully open-source. Don't let your close ones use vulnerable spyware.

It also checks all of the boxes, except what's a "foldable screen"?

Possibly a Dell XPS 13 (try to get the student discount).

https://www.dell.com/en-us/shop/dell-laptops/new-xps-13-laptop/spd/xps13dx13260laptop