What are a life tenant’s rights?

You did not mention whether this arrangement was established by a trust that was created under your mother's Will for the purpose of holding the house.  Since you state that you, yoiur wife and your sisters have various rights to the house over a period of time, this must have been set up under the terms of a trust that received title to the house unpon your mother's death.  Based on your description of the rights of you and your wife, it appears that your mother left you and your wife life estates in the house, subject to title going to one of your sisters if one of them survives you.  Assuming that my understanding of the arrangement is correct, you and/or your wife have the right to use and occupy the house as long as either of you live.  However, based on the details that you provided, it appears that you do not have absolute title to the house at this time.  A person who holds a life use of property in this fashion does not have the right to sell or mortgage it on his own.  The property could be sold or mortgaged only if all of the parties who have current or future rights in the house agree to such a transaction and sign the required documents.  You ask how much is your wife's life estate worth.  This property interest is unique to her, and cannot be sold on the real estate market.  Assumming the property rights that you describe with respect to your wife have been properly documented in your mother's Will or trust, then if your wife survives you, she will have the right to undisturbed possession and use of the house for the rest of her life.  The rights of your sister to the house would only come into being upon your wife's death.