Beatrice (Bee) Lupton? If you’re asking about the author connected to the 2010 bestseller titled "Sister," the author is Rosamund Lupton. Her novel "Sister" follows Bee as she returns home to London after her sister Tess goes missing, uncovering a web of secrets and family truths along the way. If you had a different Lupton sister in mind (e.g., a different work or a different Lupton family member), please share a bit more detail and the exact context.