Sam receives a letter from her birth mother, Rebecca Dennis. In S6 E7, when Brook first takes Sam in, Sam points out the waitress in the diner who is her mother; the waitress is wearing a name tag that reads 'Mavis'.
When Brooke and Victoria are arguing, Victoria comments that she never abandoned Brooke, and Brooke agrees, saying, "Not physically". But in actuality, Brooke's parents did abandon her to live alone when she was in high school.