Both the woman and the younger girl's dress are distinctly 1890s just before Edwardian fashion kicked in (dating this photo to at least eh 1890s and at most the early 1900s). I agree with Seoras. I think there is a good chance she is in mourning garb. Firstly, her dress isn't just dark but appears to be jet black (sometimes it's hard to tell in a black and white/sepia photo, but it's quite stark in this particular photo). Secondly, the fact that she is holding a photo also lends itself to the idea as this was popular to do in family portraits in the 1890s and 1900s when someone had recently passed away. The portrait she is holding appears to be male at the very least, based on what can be seen of the collar and neck area. I would also guess that it is an older man and not a boy based on the shape of what's exposed by the collar in contrast with what the young boys in the photograph are wearing. Also, I can't say for sure, but I think that maybe there is indeed a mustache visible. It would make sense if it was the woman's husband and the children's father, presumably having died an untimely. young death.