Sofia is married to Emilio, and they have one daughter, Isabella. Sofia has a sister named Valentina who is married to Marco, and they have a son named Luca. Marco has a sister named Bianca, who is married to Stefano, and they have no children. How is Bianca related to Isabella, and what is Isabella's relationship to Luca? Respond very briefly.
Bianca, being the sister of Marco (who is married to Sofia's sister Valentina), is related to Isabella by marriage. Bianca would be Isabella's aunt by marriage or simply her aunt. Since Isabella's mother, Sofia, is sisters with Valentina, and Valentina is married to Marco (Bianca's brother), Isabella is the niece of both Valentina and Marco, making Bianca her aunt through Marco. 

Isabella, being the daughter of Sofia, is the first cousin of Luca. Luca is the son of Valentina, who is the sister of Isabella's mother, Sofia. As the children of siblings, Isabella and Luca share grandparents, and this relationship defines them as cousins.