Alexandra's dress has a bustle, suggesting 1870s or 1880s. The 1870s bustle dresses were full-skirted, a vestige of the crinoline era, while the 1880s weren't. This looks 1880s. 1870s dresses often have square necklines, this has a vee more like the 1880s. However, Queen Victoria also has a vestige of youth in her appearance that would say 1870s. Alexandra stayed youthful and also made sure her photos were touched up to further enhance the impression. Maybe this is a photo taken in the 1880s with some pre-release retouching of the ladies.