A Boston teenager was in serious condition after she was stabbed Thursday in Dorchester, allegedly by a romantic rival, police said. Rasyelis Reyes, 19, of Dorchester, was arrested and charged in the assault, police said. Officers responded to Corona Street on a call for the stabbing just after 7 a.m., the police statement said. The 18-year-old victim was taken to Boston Medical Center, police said. According to the statement, Reyes is the victim’s “boyfriend’s new girlfriend.” Police spokesman David Estrada said Reyes was still on scene when officers arrived. She is charged with assault and battery by means of a dangerous weapon, police said.