Texas resident. Probated will
Your mother would have control during her lifetime, provided she does not commit waste and provided that she pays the taxes and insurance. Upon her death, you and your sister would control the land, or at least the 1/2 Willed to you by your father. Your mother would still be able to Will HER half of the land to someone else, if she wished to.
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The life estate owner has the full right to possess and use the land. I presume that she was left both the surface and mineral estate. The life estate owner cannot waste the land but if she owns both surface and mineral rights she can lease the mineral rights.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
Estate Planning Attorney
The life estate is transferred by what is called a life estate deed. As mentioned above, your mother has the right to use the land, as long as she does not cause waste.
As to the remainder interest, I am surprised to read that your dad "willed" the interest to you and your sister. The reason I mention this is because a well drafted life estate deed lists the remainder interest so that it would not require mention in a will. The life estate would also list how you and your sister would own the land upon your mother's death.
Disclaimer: Please note that this answer does not constitute legal advice, and should not be relied on, since each state has different laws, each situation is fact specific, and it is impossible to evaluate a legal problem without a comprehensive consultation and review of all the facts and documents at issue. This answer does not create an attorney-client relationship.