(trial court did not err in ordering defendant to pay $65,721.29, plus interest, as defendant's share of federal estate tax owed by decedent's estate; subject real estate titled in joint tenancy between decedent and defendant; defendant could not prove that joint tenancy created due to decedent's obligation to provide decedent with retirement program and that value of retirement program equaled joint tenancy interest; face of deed creating joint tenancy stated, "This is a bona fide gift and the grantor received nothing in return"; decedent required to pay pro-rated estate tax).