Over the years, a wide range of characters have been welcomed into the Marvel family:
Captain Marvel: Our hero, the world's mightiest mortal, affectionately known as "the big red cheese". When not super-heroing, Captain Marvel is really Billy Batson, child reporter for Whiz radio. Captain Marvel was apparently was modeled after actor Fred MacMurray of "Double Indemnity", who also appeared in TV's "My Three Sons" (1960-72).
Mary Marvel: Billy's sister, Mary Bromfield. Mary's surname is different to Billy's because Mary was adopted by the Bromfield family.
Captain Marvel Junior: Freddy Freeman, a paper boy in Fawcett City was crippled during a battle between Captain Marvel and Captain Nazi. To make up for this accident, Captain Marvel bequeathed a portion of his power to Freddy. Now, whenever Freddy says "Captain Marvel", he is transformed into Captain Marvel Junior. Freddy's costume is similar to Captain Marvel's, except that his costume is blue, instead of red. Captain Marvel Junior first appeared in Whiz #24 (December 1941), following the tremendous success of Captain Marvel. After initial popularity, interest in Captain Marvel Junior waned and faded in April 1953 and was not revived until 20 years later, when the entire Marvel Family was brought back into publication. As of issue #37 of the current series, Captain Marvel Junior has renamed himself "CM3" - a name that he can actually pronounce. :-)
"Uncle" Marvel: "Uncle" Dudley is Billy's guardian. In the 1987 mini-series, Dudley is truly Billy's uncle, although in other versions of SHAZAM, Dudley is merely a friend, not a relative. Either way, Dudley discovers that Billy is, in fact, Captain Marvel. For reasons that aren't always entirely clear, Dudley sometimes chooses to pose as "Uncle" Marvel (complete with red and yellow costume), despite having absolutely no powers what so ever. In the 1995 series, Dudley was granted super powers by the magician Ibis, for short time.
"Aunty" Marvel: This was actually Sivana in disguise. Sivana went to a Marvel family reunion as Aunty Minny Marvel, citing a past relationship with Uncle Marvel as his, I mean, her relationship with the Marvel family.
The Lieutenants Marvel: Tall Billy Batson, Fat Billy Batson and Hill Billy Batson were three unrelated people who just happened to be named "Billy Batson". When "our" Billy gets in trouble, the other three shout "SHAZAM" in unison, also transforming into members of the Marvel family. Reasoning that there can be only one Captain Marvel, they decide that they must be lieutenants. Needless to say, the Lieutenants Marvel are very definitely a pre-Crisis phenomena. The three lieutenants also make a very brief appearance during the Crisis mini-series. In the modern series, Mary jokingly considers the name "Lieutenant Marvel", in deference to her brother being Captain Marvel.
Hoppy, the Marvel Bunny: Hoppy is a rabbit from an alternate universe who is also a recipient of SHAZAM's power. Hoppy resides on a world populated by intelligent furry animals. When Fawcett ceased trading in 1953, Hoppy, along with many other Fawcett characters, were sold to Charlton. To avoid legal battles similar to those that forced Fawcett out of business, Hoppy became Hoppy the Magic Bunny, with a magic word of "Alizam" instead of "SHAZAM", and he was given a blue costume to replace his red one. As an addendum, in the 1987 mini-series, Uncle Dudley was a magician whose stage rabbit was called "Hoppy". Hoppy also makes a brief cameo appearance in issue #27 of the 1997 series, touted by Waverider as an example (among others) of possible but "unreal" characters who might have been (or might be) if "our" Captain Marvel's time line were not restored. In issue #29, Hoppy features in an adventure with Captain Marvel which may or may not be a dream.
Freckles Marvel: In pre-Crisis continuity, Mary Dudley (a.k.a. "Freckles") was the niece of Uncle Dudley and a friend of Mary Marvel's. Although she sometimes wore a Mary Marvel dress and called herself "Freckles Marvel", she never pretended to have SHAZAM powers and is not regarded as a true member of the Marvel Family.
Baby Marvel: Baby Marvel was a foundling who turned up on the Marvels' doorstep in Marvel Family #1. The three heroes gave the baby a red-and-yellow diaper and called the child "Baby Marvel". I'm told that, at the end of the story, the infant was reunited with its mother.