Complex sentence: Rosalba's other sister, Giovanna, and her mother, were part of the party, and both sisters, although in particular, Giovanna, helped Rosalba in painting the hundreds of portraits she was asked to do.
Split into simpler sentences: Rosalba's other sister, Giovanna, and her mother, were part of the party. Both sisters, although in particular Giovanna, helped her in painting the hundreds of portraits she was asked to do.