My understanding is that we are required to honor her will
Who are "we"? Who is nominated in the will to serve as executor? Has that person been appointed by the probate court to serve as executor?
my stepsister is insisting there is a law that overrides the will in Florida however I am not finding it.
How you phrase this depends on the extent to which you want to preserve a relationship with your stepsister, but I suggest that you pose the following to her:
"Oh...there's a law that overrides the will? Please cite that law to me and provide a link where I can read the law online. If you are unable to provide a citation and link by [pick a date], we will proceed to administer my mother's estate in accordance with the terms of her will."
Like others who have responded already, I'm relatively confident that she will be able to provide neither citation nor link.
As a practical matter, if it means that much to her, why not just give it to her and avoid the strife?
I agree with the sentiment behind this. However, if the OP's mother's will expressly leaves the item to a grandchild, then it's not up to the OP. The OP could suggest that the stepsister discuss the matter with the grandchild. If the grandchild is willing then there's no issue.