If your family can't pay the death taxes, then penalties and interest are assessed or the property must be sold at bargain prices to pay the taxes. When you create a joint tenancy you are giving a gift whether you know it or not. You immediately give up control of your property when you create a joint tenancy. With joint tenancy you no longer own or control your property by yourself. The property can be taken to pay for the other joint tenants debts and taxes without notice.
Death taxes are in addition to probate fees and expenses.
If you are not careful your family can end up paying both.