If my father and I co-owned a safe deposit box but he then passed away and my sister now wants half of the coins and savings bonds, is she entitled to them?

This may or may not be the case in Iowa so check with a probate attorney there for state law. Typically, a co-ownership of the safety deposit box does not create a co-ownership of the content in that box. Depends on how the safety deposit box account was set up, it could well be that the contents are still belonged to the estate and must be distributed accordingly.