20 Incredibly Hot Mistresses of Celebrities

17. Sienna Miller – Balthazar Getty

This one is a bit hard to swallow. Sienna Miller must not have thought about how it felt to be cheated on, because she went right ahead and started a relationship with Balthazar Getty. Balthazar was separated from his wife, but they had just had their fourth child together, and it was a bit harsh to immediately jump into another relationship with someone when his wife was busy trying to care for their newborn.