She didn't help matters much by giving each of us a capsule description and then using those descriptions almost in place of our names. I was the oldest and her 'rock of Gibraltar.' Cynthia was born 14 months later, and she was 'a joy to behold.' Ellen came two years later and she was 'a breath of spring.' Poor Elizabeth, born last and when my mother was pretty much at the end of her rope, was 'the pain in the ass.' My mother had succeeded in setting up a sisterly dynamic that would have made Deborah Tannen's eyes grow large. The Georgetown University linguistics professor who has written so many successful books about how conversation shapes relationships has just published her latest: 'You Were Always Mom's Favorite! Sisters in Conversation Throughout Their Lives.' She talked to more than 100 women of all ages, races, regions and ethnicities about their sisters, and almost every one began the interview the same way: 'We're very different.' Whether the women went on to describe a close relationship or a fractured one, they began by describing themselves in comparison to their sisters. In addition, whether the relationships were smooth or fraught, the undercurrent of competition between the sisters was unmistakable, Tannen said. 'A compliment to one can feel like a slight to the other,' she recently told an audience made up almost entirely of, well, sisters. 'It all comes down to the same thing: Who is more deserving of love.' She told the story of two sisters who were talking on the phone. One reported that she'd spoken to their mother twice that day. The other sister said she had spoken to their mother just once. '`But we had a better conversation,'' Tannen recounted. Sisters, more than brothers or even married couples, talk more often and for longer periods about more personal topics. 'You have more of a chance to connect but also more of an opportunity to say the wrong thing.' Sisters, too, are particularly good at sending -- and receiving -- what she calls 'metamessages' (we'd call them zingers) that have powerful subtexts. Birth order plays a part, too. Tannen said that oldest sisters, like me, often reported that they were accused by their younger sisters of being bossy. 'But they are expected to play a parental role. They are used to being a surrogate mother, a parentified child, and they get used to telling the others what to do.' In contrast, the youngest sister -- even as an adult -- continues to feel her incompetence. She often interprets help as criticism. And the poor middle sisters are 'a unique combination of the older and the younger.' Puzzles in their own right, who must figure out how to relate to sisters both up and down the ladder. Tannen's very talented ear allows her to see inside our most intense relationships using the window of our words. Husbands and wives, mothers and daughters. And now, sisters. Everyone who has a sister will see her sister in this book. Perhaps she will see herself as well. 'You think things should be fair and the same and equal because you were both born into the same family,' Tannen told her audience. 'But that's an illusion. It's a different family when each child is born. A sister is like yourself in a different movie, a movie that stars you in a different life.' If the truth is told, baby Elizabeth was a pain in the ass. She never took naps, refused to stay in her crib and dated a biker in high school. But she also grew up to be a nurse and shepherded my mother through her complex end-of-life care. Ellen, the breath of spring, became a doctor's office manager and the expert who managed my mother's insurance issues. Cynthia, the joy to behold, was married to a lawyer and brought him to every doctor consultation to make sure we received the attention we needed. Me? The rock of Gibraltar? I turned out not to be good for much more than comic relief at this critical time. So I'd sleep in the hospital room chair all night and bring her McDonald's for breakfast. E-mail Reimer at susan.reimer@baltsun.com. 