Postscript: There are a number of flaws with the use of twin studies to measure heritability. One is that environmental factors are variable, even for twins raised in the same environment. The second, is that modern DNA sequencing studies show that tissues are made of clonal trees defined by mutations that accumulate during cell replication as a tissue expands during development and maintenance. Hence even two embryos starting with the same genetics, do not end up exactly the same. So you may assume that different areas of the brains of identical twins can not be expected to be exactly the same genetically. Unfortunately for the science of behavior with dogs, twinning only seems to happen when done by artificial cloning. Such experiments could be quite useful for working out an estimate for heritability of behavioral traits.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2899491/