photo credit: Northampton Museums & Art Gallery
This portrait is currently listed as circle of Joseph Highmore. Might Philippe Mercier be a better attribution?
Philippe Mercier's work on Art UK: http://bit.ly/2ybsmQV
Yes. Her husband and children by Mercier are in the same collection
Mercier normally signed and normally in dark colours in dark places. . This may be discretely tucked away in the background.