As someone who has to read all those letters, I would say that unless a program requires otherwise, you should stick to letters from research mentors and a letter from someone who can write about you outside the context of the lab. More is not better, and after the fourth letter the admissions committee's interest wanes. The only exception would be if your letter writers are chemists. Letters from chemists tend to be just one or two paragraphs and are skimpy on details about your personal characteristics. If you only have letters from chemists, you need to get a letter from somone who can talk about you as a human being. (Biologists are much more susceptible than chemists to logorrhea, and will write lengthy descriptions of a student's ineffable qualities that wil "make her an asset your program.")
Letters from parents, siblings (I kid you not), sunday school teachers, etc., are not helpful to admissions committees. If you worked in your mother's lab, there is probably no way around the need to have them write a letter for you, but otherwise you should avoid anyone who cannot be expected to give a reasonably objective opinion of your abilities.
Schools may have different requirements for letters of recommendation so you should determine what you need to do for each institution. Just remember that more is not better.
