Shalom in Yeshua Ginahunt3, I am sorry a key is causing so much tension in your family. I cannot understand why it is so important to the parties concerned. The reason I say this is that apart from the memories it triggers, it is just a memorial. The souls of those buried there are not resident there.
Legally I would think that it belongs to you, but again is it worth fight over? Why not share the it.
I will pray that your all resolve you problems together.
May God bless you