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First of all, you mean that your husband had his mother's home transferred to himself where his mother retained a life estate in her home & further she stayed in the home until she died. This being the case, putting aside for the moment that a gift tax return should have been filed at the time of the transfer (no gift tax would have been due).
When a decedent retains a lifetime interest in their home after a transfer to another individual, for estate tax purposes, the home would still be included in the decedent's estate. Since unless you are dealing with an estate in excess of $5,000,000. there is no estate tax or estate tax filing due. However, the good news is that your husband's tax basis in the property is adjusted to it's fair market value at his mother's date of death. So, for example if he were to sell the home now, the capital gain would only be based upon the difference between the net proceeds from the sale and the value of the home on his mother's date of death.