No one was more surprised than I when the Sleuth Sisters series took off. What I'd intended to be a single e-book grew, due to reader demand, to include print and then audio formats. Since that first book, the sisters have returned again and again (7 times so far) with more adventures. Just about the time I think readers are tiring of Barb, Faye, and Retta, someone writes to ask when the next book will be available. "If you do decide to end this series," one fan wrote, "please don't kill off any of the sisters!" Having lost a sister IRL two months ago, I could never do that to my fictional family.
If I had to give a single reason for the success of the Sleuth Sisters, it would be sisterhood.We don't all have sisters, but those who do often recognize themselves in the stories. Readers tell me which sister they are or point out how one of the three is exactly like one of their siblings. They have strong reactions, like the woman who wrote to tell me she "can't stand Barb." Or another who wrote to tell me that she didn't like one spot in a book because "Barb is braver than that. She'd have..." and I got a long description of what Barb would really have done if someone stuck a gun in her face.
That means the characters are real to readers, and I love that. The key to writing about sisters arguing among themselves and grousing about each other is that there has to be love behind the arguing and grousing. Sisters share experiences, genetics, and such, but we aren't alike. If we're lucky, the things we share help us understand the times when we differ.