V - Twin studies are a valid way of studying nature. Since MZ twins share 100% of their genotype, but DZ twins no more than 50% but both share the same homelife, if MZ twins have similar behaviours where DZ twins do not, this is likely to be due to the genotype.
However, these conclusions are never certain. Correlations like Brendgen's do not prove causation. Something else could be affecting the MZ twins. For example, MZ twins tend to be physically indentical and get mistaken for one another. It might be that one twin gets stereotyped based on the other twin's behaviour (if one twin is naughty, both get a bad reputation). In Brendgen's study, teacher and peer ratings might have been influenced by stereotypes, with both children being given the same rating regardless of their behaviour.
As a natural experiment, this study cannot show cause and effect. For example, the twins might influence each other's behaviour, with one leading the other astray.