my husband has a child with his mistress I want to sue her for alienation of affection .he also pays her child support .

Although still technically allowed, alenation of affection suits are very much disfavored in Illinois and require, among other things, a showing of actual monetary damage from the alienation.  Punitive damages are not allowed.  There is legislation pending which would do away with this cause of action completely.  Are you sure you want to spend your time on this kind of thing?