Legal Question in Employment Law in California

Yes. That's what at-will means. They can fire you at any time for any reason other than illegal discrimination or for no reason at all, and you can quit at any time for any reason or no reason at all. Also bear in mind that it is not impossible, but highly unlikely she was fired for attendance problems because she took an approved day off. Far more likely is that this termination has been in the works for some time and coincided with her taking a day off.

No, if the motivation for the "set up" was to deny your sister statutory medical leave rights, or to interfere with a disability accommodation, or to retaliate against her for exercising some other statutory or common law privilege. Sometimes employees do not recognize the connection between the "set up" and the motive. In discrimination parlance, the "set up" is called "pretext" and the pretext is used to hide the discriminatory or retaliatory motive. For example, if an employee complains of an unsafe working condition, a month passes, and out of the blue, there is a firing for a pre-textual reason, the timing (and other circumstances) strongly suggests that the complaint caused the firing. A final note: many employers pound into their personnel policies that employment is "at will" without identifying the numerous statutory and common law exceptions to that unjust law. For more information on discrimination law, and proof of a violation, go to http://www.employee-rights-atty.com/home/we-fight-discrimination/