According to evolutionary theory on kin selection, monozygotic (identical) twins should help one another more than dizygotic (fraternal) twins.

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Monozygotic twins share all of their genes, while dizygotic twins share only half of their genes on average. According to kin selection theory, organisms are more likely to help individuals who share more of their genes, since doing so increases the likelihood that those genes (which are common to both individuals) will be passed on to future generations. Therefore, monozygotic twins should be more likely to help one another than dizygotic twins, because they share all of their genes.

According to Evolutionary Theory on Kin Selection, Monozygotic (Identical) Twins