i feel manyata is just remembering those instances and its all just to confuse us viewers that brinda was stolen that time
but i feel brinda was stolen at the time of birth only and she is the elder one so she shapeshifted first
there might be a rule that if there r twins one of them will shapeshift that day and other will after one month type some clause
and this twist came so that manyata could know she has two daughter