The reasoning for this is knowing what crossing over is and what it entails. The whole point of crossing over is exchanging the correct pieces of DNA BETWEEN homologous chromosomes. In this case, it allows for new combinations of alleles to be produced and adds to the genetic diversity of the individuals and their offspring. Because crossing over occurs between homologous chromosomes, the DNA is exchanged between NON-SISTER chromatids because it is occurring on the chromatids of two different chromosomes. Furthermore, crossing over occurs between HOMOLOGOUS CHROMOSOMES, which means it is usually happening BEFORE the homologous chromosomes are separated. Since homologous chromosomes are separated in meiosis I, crossing over also happens in meiosis I.