Alessandra Slytherin was Eerikas' mother. Alessandra is the direct descendant of Salazar Slytherin, she married a man she loved so much but the man only married her for her family name and even her family seemed to like him, so everytime Alessandra and her husband got into an argument, her family took his side.
Since Alessandra was the eldest, she was most likely to carry the Slytherin heir. They all expected a boy but she gave birth to a beautiful girl, Eerika Slytherin. Everybody was disappointed and furious that Eerika really is the heiress of Slytherin. They did have hope that she'll be evil but to disappoint them further Eerika turned out to be kind the opposite of what they expected. She even accepted muggles-borns as friends. Alessandra didn't mind it, she loved her daughter as she is, so when her family tried to kill Eerika, she did something that all of other Slytherin descendants disappeared, vanished but left Alessandra with a blood cruse.
Even though she knew that she's going to die soon, she still married Abraxas Malfoy because they loved each other so much. They got married when Lucius and Eerika was 6 years old.
Abraxas Malfoy's wife died when Lucius was only 4 years old. Lucius was quick to accept Alessandra as his mother and Eerika as his own sister, even Abraxas Malfoy loved Eerika as his own. Every thing was fine and happy until the blood cruse of Alessandra Malfoy took her life when Eerika and Lucius was 11. Alessandra never told Abraxas that Eerika was the Heiress of Slytherin. Abraxas Malfoy never stayed the same, he became strict towards Lucius and Eerika. He told them to hate the muggle borns and warned them to get in Slytherin house at Hogwarts. He even told them to be friends with only purebloods. It was easy for Lucius Malfoy to do all those things but for Eerika, no matter how hard she try she couldn't do all those things her father told them to do except to get in Slytherin house.
So Lucius and Eerika got in Slytherin house. Lucius was overprotective about who hus sister meets, talks, be friends with and all. But behind his back, she even be friends with a muggle-born and a half blood (Lily Evans and Remus Lupin) . Eerika was so talented and intelligent as well. She can play almost every position in quidditch but she fancied playing the chaser. She was also so beautiful that many boys fancied her. At school Remus was her bestest Friend.
After the death of Abraxas Malfoy, Lucius hated Eerika for he thinks that she is the reason to his death. Because Eerika didn't do what their father told them to be or do. He hated her for befriending muggleborns and Gryffindors. He hated her for loving a blood traitor and marrying him. He hated her so much that he missed her when she was murdered at Godrics Hollow with the Potters and his only niece, Y/n, leaving the Harry Potter alive with a scar.