Luna Fernanda couldn't keep Alessia due to growing suspicion of a traitor in the pack, so she sent Alessia to Fernanda.
Because of her age, Fernanda couldn't claim her as her own and came up with the story that Alessia was adopted.
When Denzel found out he had a sister, all he wanted to do was have her by his side and protect her. However, when his loved ones were being eliminated right when he turned eighteen, he thought it was better for her existence as his sister to remain a secret.
When he first visited her at the Litha Moon Pack, Alessia instantly fell in love with Denzel. It wasn't a romantic love, but just a familiar attraction. With Luna Fernanda's permission, the truth was revealed to Alessia, and she loved her brother even more.