Augusta and Thomas Hudson are everything that Susan wishes she could be. Augusta is her lifelong best friend, and the two women share a love that borders on the romantic. Thomas, on the other hand, is her first crush, and he's also the embodiment of the literary intellectual she wishes Oliver could be. As her life diverges from theirs, however, Susan realizes that they're taking a part of her identity with them.
Susan's relationship with Augusta is the most important one in her life—her relationship with Oliver included. In many ways, these two girls bond simply because they share a similar passion for literature and art. But, as Lyman notes, there's something a bit deeper going on: there is a "suggestion of lesbianism in this friendship [...] that in some early letters is uncomfortably explicit" (1.2.12). Even if you share Lyman's doubts about the sexual nature of this relationship, we can look at these hints as evidence of Susan and Augusta's closeness.
Although he's no Augusta, Thomas earns a nearly equal amount of adoration from Susan. He has pretty much everything young Susan wants from a man: sensitivity, intellect, and a finely tuned literary sensibility. But, Susan's daydreams of becoming Mrs. Hudson are cut short when Thomas falls for Augusta instead, leaving Susan to go with her fallback plan: Oliver. Susan never quite gets over this, and she becomes convinced that Augusta and Thomas look down on her for marrying a low-class guy like Oliver.
Now that that's out of the way, let's take off Susan's rose-colored glasses and look at the reality of the situation. To be honest, we never see a specific indication that Augusta and Thomas don't like Oliver; all we see is Susan freaking out because she's convinced that they hate him. We're not saying that they secretly dig Oliver or anything, but we'd wager that most of the tension between Susan's best friends and her husband exists solely in the indomitable Mrs. Ward's head.
Here's another truth bomb for Susan, while we're at it—she becomes a much bigger deal than Augusta. While Augusta ends up living a comfortable life as a wife and mother, she ultimately gives up on her artistic ambitions. Susan, on the other hand, becomes "the best-known woman illustrator of her time" (1.1.36). While this is mostly due to circumstance, we think it shows that Susan is doing just fine for herself—perhaps even better than the people she admires the most.