This example of the ‘Anna Selbdritt’ subject is an inferior version of a much-better-preserved version found here. Note the insect damage and less emotive expression of the figures!
The ‘Anna Selbdritt’ was a common sculptural theme - in fact there was a huge profusion of this combination of St. Anne with the virgin and child in the 1500s.
Tilman Riemenschneider carved several variations on this subject; see also his sandstone and other wood carvings.