Not selfish at all! My Brosca was the same way.
In fact, before Duncan, my Brosca went around rubbing everyone's nose in how she's the meanest thing with a blade (to paraphrase Leske), but Rica was the only person she was openly loving, protective, and supportive of. In fact, Brosca didn't show kindness to anyone, not even Leske, because she thought showing kindness was showing weakness. But for Rica? She openly defended her to everyone (including Beraht and Leske) and honestly didn't care if anyone thought her love for her sister was weak. She'd run her blade across the tongue of anyone that said so.
While Brosca did a lot of growing up outside Orzammar, I imagine she was still fiercely protective of her sister's happiness when she got home. Supported Bhelen without even needing to think about it since Rica seemed happy, and threatened Kala when she griped about the palace. "Rica is happy now. Don't you dare ruin that for her." (Yeah, my Brosca would not have been above committing matricide if she thought that Kala's complaining would in any way threatened Rica's position in the palace.)
God, that was a good playthrough!
My Brosca was one of my two favorite PTs. I imagined the only reason Koshka hadn't run for the surface to escape the Carta was because of Rica, and she planned to do it as soon as Rica had a noble protector. She didn't bully for the fun of it, only when she felt she needed to make certain people remembered she was a predator and dangerous.
She had no idea that there even were people as naive and sweet as Alistair, but when he turned out to be real, she decided it was her job to protect him from being used and toughen him up just enough that he'd survive without becoming bitter. When she found out who he was, she instantly put two and two together and realized that Eamon was going to want to put him on the throne, and not trusting Eamon, she decided the best way to make sure Alistair was protected was to put herself into position to be his mistress (and considering how Rica was treated by at least some of the nobles, there's no particular stigma to that for dwarves). By the time of the Landsmeet, she was thoroughly in love with him. And every king needs a fiercely protective assassin who's known to have a particular talent for poisons as a chancellor, right? (I really loved the idea of two kings with Brosca sisters as mistresses, btw, though apparently once Brosca becomes a paragon, Bhelen marries Rica.)
Did you have Morrigan along when you talked to Brosca's mother? I think it may be about the only time in the entire game when when one of your party members reacts to someone insulting you (and one of the reasons I loved Morrigan).