Translation:Only the t-shirt that she slept in is left of her.
The "her/hers" distinction is tricky and I'm not sure which of these sentences is what the original Czech was trying to mean but here's how I'd understand these sentences in English as a native speaker:
"Nothing is left of her but the shirt she was sleeping in" = she was wearing that shirt at the time that she disappeared without a trace, a bomb exploded in her house, or something similar. The shirt is the only thing remaining. The rest of her body, etc. is gone.
But she must have been wearing that specific shirt when she disappeared, otherwise it wouldn't be counted as part "of her," it would be simply considered a belonging and you'd say "of hers."
"Nothing is left of hers but the shirt that she slept in" = she is gone, and the shirt she slept in is her only possession that remains. She had other belongings, but they're all gone.
Usually when you'd say "nothing is left of him/her but" it's literally a part of the body that remains after the rest is destroyed, but sometimes it can include a thing that a person was wearing at the time they were destroyed. For example some examples from google:
"she trembles once, twice, and slides against Celia in a heap on the sidewalk, smoking like a wet fire, sweet and musky, until nothing is left of her but her fringed cotton shawl"
"Heartbroken, Echo spends the rest of her life yearning for Narcissus in lonely glens. She eventually wastes away until nothing is left of her but her voice"
"Mombi is ruthlessly doused with water and melts away like the Wicked Witch of the West, so that nothing is left of her but her buckled shoes"
Vs. "nothing of hers":
"However she has taken with her the things one never leaves behind: almost nothing of hers remains in the bathroom and my toothbrush is alone again"
"Hatshepsut's tomb was probably looted soon after it was sealed. It was never found and nothing of hers remains in the world"
"Nothing of hers remains inside the house"
But we have "zbývá z ní", that is not just a belonging somone left somewhere, that means it is a remaining part her.
To be sure I opened the national corpus and search for "zbývat z" and I am pretty sure it really must be that it is the only part of the person that remains.
For some belonging left behind you would use "zbývá (tu/tady) od ní...".
My take, FWIW: The subject of the Czech sentence is not "she," it is the t-shirt. "She" has somehow mysteriously disappeared, and the only thing left to show that she was ever there is the t-shirt in which she was sleeping. If "she" were the subject, I think we'd have a different sentence in Czech.