How could she? That’s what the sister of alleged killer and Upper West Side nanny Yoselyn Ortega said in a new interview from her home in the Dominican Republic. Mylades Ortega said she didn’t understand how her sister could have done what cops allege: that she stabbed two children in her care to death before attempting to end her own life. “This isn’t the Yoselyn we know,” the nanny’s sister said. “She loved these children like her own.” Leo and Lucia Krim were found dead in their family’s West 75th Street apartment Thursday.