I am probably in the minority, but I adore the relationship between Sebastian and Viola. Of the siblings, they are the most alike, though there are some marked differences. Amy's death split them, but there were cracks that started long before Elrachaim killed her. Viola wanted to break with her siblings before that showdown. Sebastian wanted to stop searching for their father and settle down to start a life with Amy. They were drifting apart and poor Olivia was stuck trying to hold them together.
In The Chaos Child, Sebastian and Viola try to work out some of their issues but things aren't easy because of his Dreamless Sleep addiction and the matter of trying to rescue Olivia. In the end, she tells him that she knows he'll never trust her again and that they won't have the relationship they once had. He surprises her by sticking around, and I think he'll surprise you in Mistress of Malice and Mercy.
I also adore their teasing - it's not quite the same as her banter with Duke because there is an edge to it, but there is always affection. This is one of their scenes together in MMM:
Tears stung Viola’s eyes. The lump in her throat hurt and she wanted to bawl like a baby. She flung her arms around Voltnor’s neck and dragged him down into a bear hug. “I love you guys so damn much.”
“Whoa there, little sister,” Sebastian teased. “Let’s not say things like that with Duke around. I’m not sure who I’d pick in that fight.”
She spun around and threw herself at Sebastian. He stumbled back a few steps but quickly regained his balance. Her arms and legs were like vices around his neck and knees. “I love you, too.”
“Hormones?” Sebastian asked an amused Voltnor.
”I would not dare presume to...”
Viola reared back to glare at her brother only to freeze when she caught a glimpse of his black eye and busted lip. His left cheek was puffy. Still clinging to him, she arched an eyebrow and drummed her fingers on his shoulderblades.
“What in the hell happened to you? Sweet mercy, Seabass-the-stupid-ass, who did you piss off this time? I have warned you about getting snippy with the Yvaltas in the cabin across from yours. How many times do I have to tell you that you cannot, I mean cannot, tease them about their horns. Did you just stand there and let whoever it was kick your ass or did you at least try to defend yourself?”