The Crawford siblings, though sympathetic in many respects, are antagonists because they oppose our protagonist, Fanny. What Fanny wants more than anything is for Edmund to fall in love with her, but Mary Crawford marches on to the scene and steals his heart. Henry Crawford also gets in the way of Fanny's dreams by pressuring her to marry him. To make matters worse, Edmund also tries to convince Fanny to marry Henry. It's not until the Crawfords leave for good that Fanny's dream of being with Edmund can be realized.
Mrs. Norris is pretty straightforward as an antagonist. She's always mean to our protagonist, Fanny. She causes a ton of problems for everyone in the book. And no one likes her.
Sir Thomas is a bit harder to pin down. Granted, he causes a lot of problems for people. And his kids see him as an antagonist, which is important. Image may not be everything, but it is something. Sir Thomas comes across as cold, serious, strict, uncaring, etc. His strict attitude leads his kids to rebel disastrously. And much of Fanny's timidity results from her fear of him.
But Sir Thomas is sympathetic and actually well-intentioned. By the end of the novel he has transformed into a much more likable person. So he's a part-time antagonist.