INAL but as the records are already in her possession it's up to those suing her to prove that she has wrongful possession of them. OP should also be able to find witnesses to state that the aunt had a verbal will, that the records were willed to her, that she took possession of the records before the aunt passed and that the matter went uncontested for several weeks. The willing to testify to that should include OPs relatives, and at least in part including those suing her. If they don't, then they are risking their own inheritance.

Also, they'd have difficulty making a good legal argument for selling and splitting only a part of the aunt inheritance, and leave everything else unappraised and unshared. If the judge agrees that the inheritance should be sold and split equally amongs the heirs, then they would insist that it went for all the ingeritance and the crystal figurines, persian rug, jewellry and everything else would have to be sold of and shared as well. If they live somewhere where heirs aren't legally intitled to equal inheritance it will also be a difficult argument to make for them. Likewise their argument that the records are especially valuable are weak as they only know of their value from someone who heard it from someone that heard it from someone else, and that person isn't even a professional. If they state that in court, they are also themselves admitting that the tecords were in OPs possession and uncontested, which should matter a lot here.

It's should legally harder to take something that's already in someone elses possession than contesting a will. Especially as they don't have an argument for why they should be able to take it that doesn't risk everyones inheritance.