These are all of the SquarePants family members, as the oldest known family member so far is the Primitive Sponge, and the youngest is technically SpongeTron and his clones.
Also listed is the relation to SpongeBob.
Primitive Sponge - He is for now the oldest known family member of the SquarePants family, as he is older then SpongGar, which is obvious, as he wears less clothing, and only makes howls, and grunts primarily, as the same goes with the Primitive Starfish, who is the oldest known member of the Star family. Primitive Sponge has only appeared in one episode which was "SB-129", when Squidward accidentaly traveled to the future and met SpongeTron and his clones, then traveled to the time where the Primitive Sponge was in.
SpongeGar - The second oldest known family member of the SquarePants family, he is much more intelligent then the Primitive Sponge. As he wears more clothing, has a language, and lives in a cave-man version of SpongeBob's house. He also owns a pet, who's name is Gary, and most people call SpongeGar's pet "Prehistoric Gary". SpongeGar first appeared in the episode "Ugh" as one of the main characters in the episode, he also made a cameo apperance in the episode "Atlantis SquarePants". SpongeGar is shown to be friends with Patar and Squog, which are the ancestors of Patrick and Squidward.
SpongeBuck SquarePants - SpongeBuck SquarePants is the third oldest known family member of the SquarePants family. SpongeBuck is from the 1800's and is known as the one who saved Bikini Bottom (at the time One Eyed Gulch) which was in lead of Plankton's ancestor, One Eyed Plankton. SpongeBuck saved One Eyed Gulch, and the town was renamed to Bikini Gulch and later on to Bikini Bottom. SpongeBuck also has a distinctive accent, which sounds like a southern accent, as it was shown he moved into (at the time known as One Eyed Gulch). Meaning that some time between SpongeGar's life and before SpongeBuck's a SquarePants family member moved out of Bikini Bottom, and SpongeBuck was the one to put the Squarepants family back in Bikini Bottom.
Grandpa SquarePants - As the possible 4th oldest SquarePants family member (as its possible Grandma SquarePants is). Grandpa SquarePants has only been shown in the series twice, once when SpongeBob imitated his grandfather, the second time when SpongeBob kept thinking about Grandpa SquarePant's wise sayings. Grandpa SquarePants himself has never actually appeared, as its possible he may be deceased or him and Grandma SquarePants are divorced.
Grandma SquarePants - The possible 4th oldest SquarePants family member (or 5th oldest). She is shown to be very caring for her grandson SpongeBob SquarePants, as she hasn't been shown with any other interactions with her grandchildren besides SpongeBob. As said above, its possible her and Grandpa SquarePants are divorced or she is the widowed wife to him. Grandma SquarePants is generally shown to be very kind and friendly (except for one instance when he got an Abrasive side).
Blue SquarePants - Blue SquarePants is the oldest uncle of SpongeBob and his cousins. Blue has only appeared in one episode, when he was visited by SpongeBob, as SpongeBob wanted him to help him with Blackjack, though Blue didn't lsiten and instead thought SpongeBob wanted to do chores for him, which SpongeBob did. He is shown to be fairly scared of Blackjack SquarePants.
Harold SquarePants - The father of SpongeBob SquarePants. He along with his wife Margaret, are shown to be very caring about their son SpongeBob. It is also shown he and his wife are wanting SpongeBob to pass his boating test so he may be able to finally drive. Harold and Margaret both always appear together in any episode they do appear in. Harold is also the brother of SpongeBob's uncles Blue SquarePants, and Sherm SquarePants.
Margaret SquarePants - The mother of SpongeBob SquarePants. She along with her husband Harold, are shown to be very caring about their son SpongeBob. It is also shown she and her husband are wanting SpongeBob to pass his boating test so he may be able to finally drive. Harold and Margaret both always appear together in any episode they do appear in. It also shown that Margaret's maiden name is Bubblebottom, revealing that SpongeBob's other grandparents last names are Bubblebottom.
Sherm SquarePants - Sherm is the youngest uncle to SpongeBob and his cousins. He has never physically appeared in an episode, though a picture of Sherm has been shown. Sherm is said to be very famous in the family, for being able to stick a watermelon up his entire nose, Sherm resembles his nephew SpongeBob very closely. Sherm is also the father of Stanley SquarePants one of the cousins of SpongeBob. His son Stanley is also the tallest of all SquarePants family members.
BlackJack SquarePants - The bully of the SquarePants family. He is shown to have mainly bullied SpongeBob when they were younger, as SpongeBob and Blue are both shown to have a fear of BlackJack. As BlackJack used to be tall, but when he was put into jail and got out, he was shown to have gotten much smaller in height, and was smaller than SpongeBob's foot.
SpongeBob SquarePants - One of the main characters of the show. SpongeBob is the most known of all SquarePants family members in the show. He also appears in every episode of the show (as he is the titular charcater of the series). SpongeBob is always shown to be friendly (though sometimes he can get angry, like when he got mad at Mr. Krabs for a dime). SpongeBob and Patrick are shown to be best friends, as sometimes they are shown to have met as infants (which was seen in a short). SpongeBob resembles most other SquarePants family members as well, and has a toatal of four (4) cousins, and two (2) known uncles, and at least four (4) grandparents who two of which (Margaret's parents) have never been shown.
Stanley SquarePants - One of the cousins of SpongeBob SquarePants. He is the tallest family member shown of all the SquarePants family. Stanley is also accident prone, as any thing he touches will explode or be destroyed. Stanley because of him being accident prone, also has a fairly hard time trying to get a job (though SpongeBob got him a job at the Krusty Krab, and then at the Chum Bucket so he could destroy the Chum Bucket).
Todd SquarePants - A cousin of SpongeBob Squarepants. He has never directly appeared in the show itself, only in a book. It is unknown if he'll ever actually appear in an episode.
Larry SquarePants - The youngest cousin of SpongeBob SquarePants. He has never appeared in the show or a book (though Todd did appear in a book), Larry was only mentioned in a video game, so little information about him is known.
SpongeTron X, Y, Z- The clones of SpongeTron, they could be considered as his "offspring". They briefly appear in the episode "SB-129."