Considering the fact that Pippa Lee is being played by Robin Wright Penn, Bello is a great fit. She looks more like the other actress, and is a bit easier to buy as a relative. As for Gyllenhaal -- the scheduling doesn't interfere with other work, but rather, her family time. She's chosen to spend this time with husband Peter Sarsgaard and their child while he films that lascivious UK movie, An Education. Or, maybe she's just keeping an eye on things to make sure that sexual education doesn't jump off the set? (I kid!)
The film focuses on Pippa, a woman whose husband falls for a younger woman and leaves her in the dust. Instead of just falling apart, she explores her "buried sensuality" and then falls apart. You can get all the other casting bits and details in our previous posts here and here.