What Do I Do?

My sister (28) rents a home with her 2 daughters (3 & 10). I helped them move out of their rental (which we will call home A) to their first ever home (which we will call home B) about 2 weeks ago. When she moved, we spent hours cleaning home A. You know how trailers have small pieces of vertical wood that line the walls, one came off while we were moving. She replaced 3 blinds in the living room because they were broken, lastly she put aside $1,000 for the carpet knowing it would need to be replaced. My sister had let the renters know about these things when they happened. After she moved into home B she received a letter from the renters of home A. Long story short it was a list of things “wrong” with home A after she left. Like, the replaced blinds being .5inch short, the lightbulb in the guest bath being taken (who tf takes a lightbulb), and so many other things. At the bottom of the letter was the amount she is being charged with a little note stating she has 10 days to pay it. It’s a little more than $2,000. Is there anything she can do to fight this? The renters state the trailer is already cleaned and we didn’t take pictures (we learnt our lesson). Always take pictures if you are renting. Anyways any advice would help!!