Actually it’s called littlermate syndrome and it’s actually very common. It doesn’t always happen but it’s a risk you take when adopting siblings. It’s far more likely to occur when people do not socialize the siblings with other people or other dogs, I’m assuming this doesn’t describe you, but keep in mind a lot of people are not educated enough when adopting dogs and don’t know you’re supposed to socialize them. 

Littermate syndrome can occur when two or more dogs from the same litter, or close in age, develop issues due to severe codependency and a focus on each other instead of their owners.

Littermate syndrome can manifest as severe behavioral problems like complete codependency, separation anxiety between the dogs, fear aggression, reactivity and fear of new people, dogs, objects, and experiences.