Maybe not. Here is a fascinating 2011 paper (pdf) from Robert Plomin and Denise Daniels, and here is part of the abstract:
…environmental differences between children in the same family (called ‘‘nonshared environment’’) represent the major source of environmental variance for personality, psychopathology, and cognitive abilities. One example of the evidence that supports this conclusion involves correlations for pairs of adopted children reared in the same family from early in life. Because these children share family environment but not heredity, their correlation directly estimates the importance of shared family environment. For most psychological characteristics, correlations for adoptive ‘‘siblings’’ hover near zero, which implies that the relevant environmental influences are not shared by children in the same family. Although it has been thought that cognitive abilities represent an exception to this rule, recent data suggest that environmental variance that affects IQ is also of the nonshared variety after adolescence.
My favorite part of the paper is the section which discusses how siblings represents a non-shared environment for each other. For instance my sister grew up with a slightly younger brother and I grew up with…a slightly older sister, which is a somewhat different proposition.
For the pointer I thank Michelle Dawson.