mine are
cory matthews & morgan matthews form boy meets world
kevin arnold & kearn Arnold form the wonder years
micheal kenton & jennifer kenton from family ties
richie cunningham & jonnie cunningham form happy days
eddie & laura wilson from family matters

Frasier and Niles Crane (Frasier)
Roseanne and Jackie Harris (Roseanne)
Ross and Monica Geller (Friends)
Darlene and Becky Conner (Roseanne)
Bart and Lisa Simpson (The Simpsons)
Dipper and Mabel Pines (Gravity Falls)

Sue, Axl & Brick Heck (The Middle)
David, Mary, Joannie, Nancy, Susan, Elizabeth, Tommy & Nicholas Bradford (Eight is Enough)
Richie & Joanie Cunningham (Happy Days)
Frasier & Niles Crane (Frasier)
Keith, Laurie, Danny, Tracy & Chris Partridge (The Partridge Family)
Greg, Peter & Bobby Brady - Marcia, Jan & Cindy Brady (The Brady Bunch)

Phineas and Candace Flynn from Phineas and Ferb

The Brady Bunch Kids

I think the most believable siblings on any sitcom were Wally and Beaver on "Leave It To Beaver and Frasier and Niles Crane on "Frasier" Whoever cast them as brothers knew what they were doing.

The Brady Bunch kids
The Eight is Enough kids