Hi, my daughter is about 15 days old and she has a yellow blister on her lip, i suspect it might be a cold sore is it possible?

Unlikely: These are usually feeding blisters from vigorous rooting/sucking. Fever blisters are a secondary recurrence of a deadly virus in infants that age & occur at the skin margin or outside the lips. The kid is not old enough for this to be the cause.

Life's possibilities: Many things are possible in life. The true question is, "Is it likely?" Oral Herpes-1 infections happen to the majority of people by adulthood. A new baby should not be kissed by herpetic adults (many adults have it) until she is old enough to tolerate a herpes infection. A baby should be checked, if signs of herpes... but a single lip blister in a neonate is more likely due to sucking friction.