I agree that it appears to be the same young woman in all four photos. I also agree that all four photographs appear to be from the mid to late 1890s based on wardrobe and mount. It appears that you have to pairs of photos, each pair apparently taken around the same time, probably during the same setting.
I’m inclined to say that the photos with “17 or 19” written on the back of them were taken a little earlier than the headshots. When comparing them, it appears that the nose and muzzle area of the face are extended further out and that the top tip of the ear is a bit elongated, indicating growth. While the faces are not in the exact same pose, and even a small degree of difference in angle can make a difference, I’ve overlaid the two images so you can see what I mean. The red lines are of the girl with the white lining, the blue lines the woman in the headshots. Using the left eye and ear as an anchor point, it appears to me some of the features in the photo when the girl is 17 or 19 (red) are a bit more close together than the headshots (blue).
Aside from this I would say that the headshots show a slightly more mature style. The dress seems more adult to me as well as the hair. I don’t believe an adult woman would let her hair down, even partially, as seen in the photo of the 17 or 19 year old.
I would guess you’re looking for someone born between about 1877-1885. In my experience, these sort of group photos of younger folks in the late 19th century weren’t always siblings. Sometimes they were close cousins and sometimes just friends. If your ancestor and his sister didn’t have any other sisters born around this time, I would take a look at their first cousins next.
