It feels strange to leave out Abby Fairgate, but I decided to vote for those characters who never stopped being someone's sister.
1. Kristin Shepard (Dallas)

Because she wanted to have everything her sister had, then she shot it, and then she drowned in her sister's pool. Her legacy lives on when her child is being adopted by her sister's brother-in-law. I think that ranks high on the soap-o-meter.
2. Terry Hartford (Falcon Crest)

I think it's the contrast that made it so interesting: how could the show's heroine and this conniving, heartless gold digger be sisters?
Actually they did stop being sisters when it was revealed that Maggie was adopted, but at least they grew up as sisters. And she married her brother-in-law's cousin, so they kept it in the family anyway.
3. Caress Morell (Dynasty)

If I had to vote for the most fabulous sister then she would have been my #1. It was a delight to watch her scheming, lying and blackmailing her way into Carrington and Colby territory. Sister Dearest, however, was not amused.
4. Katherine Wentworth (Dallas)

She wasn't that bad right from the start, but circumstances and an obsessive love turned the family relationships a little sour, as it were.
Of course it's all Bobby's fault, he was just too sexy. Katherine = justified!
5. Rose Hollister (The Yellow Rose)

This was a double role, because the was the villain's sister and also someone's previously unknown mother.
Nevertheless, her brother killed her.