Illegitimate children also often appear in the father's will or in grandparents' wills.
For example, there was a Charles Joughin we couldn't place until, entirely by accident, I came across his grandmother's will. "Itm She left to her Daughter Margt's son Charles Joughin Thirty shillings when he will go to a Master Trade." See: http://www3.telus.net/lawson/twill/1788_019.html
When looking through reels of film for a particular will I always looked through every will in the parish, and found numerous siblings and aunts and uncles who often confirmed the family group and added a lot of extra information.