Thanks for everyone’s comments and suggestions. It makes this puzzle more interesting!
As background to these photos – I inherited a family bible belonging to my great grandparents, George and Janet Wightman, both born in 1857. They have written a number of family birth, marriages, deaths in it, and included a lot of photographs. Also in the bible was an original of their marriage certificate from 1878 and two portrait paintings which I’ve added below. My assumption was that the portraits were of my great-grandparents (although it’s possible that they were given to them by someone else). The two portraits are not in the best of condition, and I’ve been told that this is because they were painted directly onto photographs. The chemicals in the photos and in the paint are now reacting, and deteriorating.
As they were painted on photographs, I was able to look through all of the photographs I had to see if any matched the two portraits. Only two were similar. The first one in the original post above, which I assume is Janet, and the one below, which I assume is George (it’s not a great match, but no other photograph in the set is even close to the portrait image). The back of George’s photo is similar to the other photos with an MIP “” code.
So the 1879-1880 dates for the photos suggest that these two photos (and the portraits) were taken shortly after their marriage, and they are both around 22 to 23 years old. I think that fits the photos.
I had wondered if the older woman was also Janet, mainly due to the same jewellery and somewhat similar looks. But if this photo was taken around the same time as the others, then I agree with the comments above, it’s probably a relative.
Janet’s mother was born in 1824 which would make her 55 in 1879. I think the older woman in the photo is therefore unlikely to be her. Janet had an older sister, Lydia, born in 1852. So she would be 27 in 1879, and a possible candidate.
I've got a family portrait which might show Janet and George at a later time with 4 of their children, which I'll post later.
Ian