Mariana Castillo grew up with Veronica Lodge and her family. Their parents were in the same line of work together, automatically making it to where Veronica and Mariana grew up together and become as close as sisters. Not only that, they are cousins.
Mariana's mother, Alayna, is Hiram Lodge's only younger sister. Then she married Lucas Castillo when they had Mariana at a very young age.
Teen parents.
Hiram took Lucas under his wing, knowing that they needed income for their new family they were starting. Hiram was more than happy to provide for the baby. After all, the baby was blood. His baby niece. A baby girl born just after his own.
Mariana was upset when her Uncle Hiram was hauled to jail right in front of Mariana and the Lodge women, but she was crushed when she heard that Veronica and her Aunt Hermione had to leave New York to a small town where their parents were from.
Riverdale.
Finally, a couple of months of them being apart, the Castillo family got word. Uncle Hiram was let out. And he was headed to Riverdale to reunite with his wife and daughter, followed by the Castillo's themselves.
(I DON'T OWN ANY OF THE CHARACTERS EXCEPT FOR THE CASTILLO FAMILY.)