her clothing looks more adult, almost like a widow's cape
She is wearing "la coiffe brabançonne", a black cloak over a white dress, a fashion that was popular in the French-speaking domain of Brabant, Belgium, during the XIX century.
The painting that Katenka posted is the original one, painted in Paris in 1844, and now in the Royal Collection of Belgium. The image was reproduced for royal relatives, and also circulated in other formats, as the one Rani posted.