Can a 12 year old girl get breast cancer?

Breast Cancer Rare: Primary breast cancer is rare before age 25 yrs. When breast cancer is seen in an adolescent, it is frequently caused by spread from another site in the body or from structures within the breast such as tumors of blood vessels. However, onset of periods before 12 yrs and prior radiation treatment near breasts is linked to adult breast cancer in teens. Consult your child's doctor if a mass is felt.

So extremely rare: I am not going to say "never", but the incidence is close to that. Anytime there is breast tissue present, there is a theoretical risk of breast cancer. However, lumpy breast tissue is an early adolescent is so much likely to be something else - most commonly fibrous breast tissue. If there is extreme worry a simple ultrasound could set mind at ease, but it is probably overdoing it!