Classic Cartoons:
* The Flintstones: Fred and Wilma have daughter Pebbles, and Barney and Betty have son Bamm-Bamm
* The Jetsons: George and Jane have daughter Judy and son Elroy.
* Scooby-Doo: Shaggy and Scooby are best friends, practically brothers.
* The Smurfs: There are many Smurfs, with Papa Smurf being their father figure.
* Tom and Jerry: Though not explicitly stated, they could be considered siblings.
Modern Cartoons:
* The Loud House: Lincoln is the only boy in a family of 10 sisters.
* The Powerpuff Girls: Blossom, Bubbles, and Buttercup are sisters who fight crime.
* Adventure Time: Finn and Jake, while not biologically related, are best friends and consider themselves brothers.
* Gravity Falls: Dipper and Mabel Pines are twins who go on adventures in the woods.
* Steven Universe: Steven is a half-human, half-Gem who lives with his three Gem mothers.
* The Amazing World of Gumball: Gumball and Darwin are brother and adopted brother.
* Amphibia: Anne Boonchuy is a human girl in an amphibian world who befriends the Plantar family.
* Big Mouth: A group of friends navigate the trials of puberty together.
Other Examples:
* Recess: The gang of schoolyard friends are all close and act like siblings.
* Rugrats: Tommy, Chuckie, Phil, and Lil are all best friends and play together like siblings.
* Hey Arnold!: Arnold and his friend Gerald are close and often act like brothers.
* The Simpsons: Homer and Marge have three children: Bart, Lisa, and Maggie.
This list is not exhaustive. There are many other cartoons that feature sibling relationships, and you can find more by searching online or asking for specific examples.