Is it permissible for me to marry my paternal cousin's daughter, given that her mother breastfed me along with her younger brother?

If you were suckled by the man's mother five full sucklings within the first two years of your life, he becomes your brother through suckling (rada'a), and his daughter becomes your niece through suckling. It is not permissible for you to marry her, because the Prophet, peace and blessings be upon him, said: "Indeed, suckling prohibits what birth prohibits." Your not being suckled with him does not affect the ruling; rather, what matters is that you were suckled by his mother.