Your mother can leave her estate to anyone she wants, even non-relatives. Be sure she goes to an attorney so that her will is pepared and signed correctly. Your sister can always complain that things aren't the way she wants them, but so long as your mother's will is done correctly your sister's success in contesting a will under the circumstances you describe is about zero.
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Attorney Lindquist offers sound advice. So long as mom is legally competent she can craft her estate plan in any fashion she so chooses. You sister needs to mind her own business. Mom should seek the services of an estate planning attorney to have her will and trusts and other estate planning documents set up as she chooses. Mom may also consider an "in terrorem" provision that would invalidate a bequest to someone who challanges her estate plan. Get her to a good estate attorney. By the way take her if you must but let her talk with the attorney alone.
For more on estate planning and other issues, see Estate Planning Mistakes: 5 Not So Easy Pieces at http://www.sjfpc.com/estate_planning_drafting_wills_trusts.html. Please hit the like button at the end of the article if you found it helpful.
For more on talking with aging parents about this topic please read Estate Planning For Elderly Parents: Discussing Finances and Estate Planning with Your Aging Parents at the following link:
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Your mother should seek out an attorney who practices in the field of estate planning. He or she can assist her with preparing an estate plan that meets her wishes. Your mother can divide her assets however she chooses. There are limited grounds for contesting a will. A will can be contested because of undue influence, lack of capacity and lack of proper formalities. As the previous attorney mentioned, it is not likely your sister would be successful in contesting the will, under the circumstances you described.
As the other attorneys responding to your question have duly noted, your mother's intentions need to be properly memorialized in a Last Will and Testament. It is her wishes that matter and not those of any family member. One attorney mentioned an "in terrorem" clause, which is basically a threat made to any beneficiary thinking about contesting a will. In Florida, "in terrorem" clauses are not valid, but that is not relevent as long as your mother is mentally competent, is not being unduly influenced by another, and has a Will prepared and properly executed. Any meeting that she has with an attorney should be outside of the ears of any family members or friends. Confidentiality between the attorney and his/her client is extremely important, especially inn a situation such as this.