Since you're living there with your mom's knowledge and permission, adverse possession (also called squatter's right) probably would not apply in your situation.
You need to have a very frank talk with your mom and get her to have a will drawn up and put in place ASAP so her wishes for her property can be legally enforceable after her passing.
You can get a very simple (and legal) will form at
http://www.uslegalforms.com/. Why not buy a copy, take it over to her, and talk to her about it. You might also consider getting your siblings on board with this while your mom is still alive. These wills are state specific so be sure and get the one for Arkansas.
If she dies intestate (without a will) the state has a predetermined forumula to divide her assets to her heirs.