You can physically be anywhere in this world when you make your will. But legally, the document will be interpreted according to the laws of the will maker's state of residence, which should be clearly stated in the will.

Also, if your mom-in-law opts to make her will self-proving -- a procedure that avoids the need to prove the will in a probate proceeding after death -- she should use an Illinois notary public on the self-proving affidavit.

Using will-making software will work just fine and it won't change the residency situation one whit. The best software -- for example, Quicken WillMaker Plus and Nolo's Online Will -- will produce a document tailored to the laws of your mother's state of residence. So your mother's will should be tailored to the laws of the Land of Lincoln, but she does not have to be in the state when she writes it up. Honest.