@lovethesun: given your history of having an existing contentious relationship with your SIL, why does her ignorance and grudge holding even emotionally bother you? Why does it surprise you? Just ignore her. Don't let her social media declarations of godliness annoy you as much as entertain you.

Like the others here said, people hold grudges for many different reasons: narcissism, delusions of grandeur, religious reasons, boredom, the need to dominate others. People who hold grudges against someone do it, because they have an unmet need that the grudge fulfills.

Maybe your SIL is really anxious, angry, depressed or sad about her relationships, her business, or her life overall. Or maybe she's just a mean person and excessive grudge holding is akin to an Olympic sport to her. She thrives on it. Who knows.

Just ignore her. For your own well-being.