Princess Stella ain't exactly front and center in this book. In the few times that she's mentioned, she's always described as an emotional, loyal wife who loves and respects her husband almost to a fault. The book shows her submissiveness toward Prince Fabrizio,
[Her] fine crazy eyes glanced around at her slaves of children and her tyrant of a husband, over whom her diminutive body yearned for loving dominion. (1.6)
Even after her husband has behaved like a cheating jerk, she takes comfort in feeling his large, masculine presence next to her in bed: "Stella lay back too, and as her right leg grazed the left leg of the Prince she felt consoled and proud at having for a husband a man so vital and proud" (3.15). What she really needs to do, though, is demand some R-E-S-P-E-C-T.
Apart from her faith in her husband, Princess Stella has a deep commitment to religion, even though she sometimes seems more invested in habits than in actual faith. As the text says,
Maria Stella did not say a word at first, just made a series of signs of the Cross; then she remarked that she should have crossed herself with her left hand and not with her right; after this supreme expression she loosed the thunderbolts of her eloquence. (3.10)
When she forgets which hand to use when making the sign of the cross, she shows us that she's more preoccupied with making the sign than thinking about what it actually means. But giving into superficial gestures instead of thinking deeply about them is nothing new for the aristocracy.