There are two broad categories of assets which are includable in your gross estate which is subject to tax: property you own at the time of your death and certain property transfers you make during your life. See Internal Revenue Code Sections 2033-2044.
The effect of these Sections is that property you own jointly with another individual will be taxed at your death, even if you did not purchase the property or do not have complete rights to the property.
For example, if you and your spouse own an asset as joint tenants with rights of survivorship and you die first, at your death one-half of the value of the asset will be included in your taxable estate. Your estate will include this one-half value regardless of how the asset was acquired or who paid for it.
A second example is where you own the asset with your sister (or anyone other than your spouse) as joint tenants with rights of survivorship. If you die before your sister, the full value of the asset will be includable in your estate unless you and your sister received the asset as a gift or your sister paid part of the purchase price for the property when it was acquired.
Consult your estate planning attorney for further information.