It's a tough situation - I agree that Brandy sucks for chasing her sister's boyfriend.
I have an older sister who was always the one who got all the boys, etc. and while we were far enough apart in age to avoid crossover boyfriend issues while I was growing up, we're close enough in age that I layed out some ground rules as an adult:
- I will not date anyone who has ever had a thing for my sister.
- Anyone who has ever had a thing for me generally will not have a thing for my sister; we're very different women in terms of looks and personality.
My first instinct is to say that if my sister did what Brandy did, I'd show no mercy. But that's not really true. I love my sister; she's family, and boyfriends are not family and ultimately replaceable. But you can't replace your sister.
So I'd just guilt trip her and bring it up at family reunions every couple of years as a diversion when my family starts pestering me about my lifestyle choices. All's fair among sisters, after all.