Lethe is actually a fair bit older than Adrianna (their specific ages are in a previous answer here somewhere) though their relationship ended up with her in a more assertive, older sister sort of role once they were both older. They have the same father, who was not a mage. Lethe's mother died when he was young, and Adrianna's mother ditched when she was a kid, so it's mainly been all about their dad who raised them both alone. As for which of Adrianna's parents was a mage, the answer is neither though there was probably something recessive inherited from her mother. Adrianna's just one of those (unfortunate?) genetic anomalies. It's not a common occurence, but it does happen.