Why Is This Woman Pinching the Nipple of the Other?

Decoding a painting of two sisters in a bathtub

Gabrielle d’Estrées and One of Her Sisters (c. 1594) attributed to the School of Fontainebleau. Oil on oak wood. 96 × 125 cm. Louvre, Paris, France. Image source Wikimedia Commons

It is hard not to be intrigued by a painting like this. It shows two women sitting unclothed in a bathtub. The one on the right is named Gabrielle d’Estrées. Beside her is another woman — thought to be her sister — who is unambiguously pinching her nipple.