Can a family member contest a living will that is witnessed?

I assume that you mean a last will. A living will normally is used as another name for a health care directive.

In either case, a person can contest either if they have a legitimate interest. For a living will, that will typically be the next of kin. For a last will, a person must either be an heir at law (roughly the next of kin) or a beneficiary under a previous will.

If the person has children, the nieces and nephews would not be either heirs at law or the next of kin.

A will must be witnessed to be valid. However, there are other requirements for a valid will (mostly dealing with the mental state of the person signing the will). If his nieces and nephews are either heirs at law or were included in the previous will and think that their uncles mental state was poor at the time that the will was executed, they should talk to an attorney licensed in Illinois to see if they have a basis for challenging the will.