Is this A World Without You...?
After being taken in by the noble house of Nacht, Sienna grew up with her adopted sister, Lorena, who she believed to be the sweetest, kindest person in the world. But after Lorena attempts to kill her, Sienna realizes how much her "loving" sister actually hated her. She tries to end her life with a magic potion but ends up going back in time instead. Determined to survive, she tries to use her memories to her advantage. For some strange reason though... Lorena is now nowhere to be found.
The story is set in a magical empire which is at war with demons. Magicians are a valuable resource in this war, and as such, it's common for nobles to adopt children of all backgrounds if they have magical talent.
The female lead, Sienna, is an orphan with brown hair and green eyes who lived in an orphanage for children with magical aptitude, until she was adopted by the Grand Duke, Rodrik Nacht, who has black hair, red eyes, and two biological sons, Asiel and Mihael (who also have black hair and red eyes).
When Sienna arrives at the Duke's mansion, she meets Lorena Minanchy, an older blonde girl from a noble family who was also adopted by the Duke. Lorena is outwardly sweet and charming towards Sienna, but uses deception to steal for Sienna's magical power for herself. Sienna later ends up in prison, having been framed by Lorena, and commits suicide by drinking a vial of poison.
She then wakes up as her younger self back in the orphanage, on the day that she was adopted by the Duke in her previous life. She resists being adopted by the Duke this time, in an effort to avoid living with Lorena again, but her plan fails and the Duke adopts her anyway.
You can read this manhwa at Tapas.