Smoking is a risk factor for cancer, heart disease, and other serious medical problems. Period. It is very unfortunate that your daughter has decided to adopt the habit, and you are right to be concerned about her health, both in the long term and in the short term. Additionally, smoking while you are young is even more harmful, as your body is more susceptible to damage while it is actively growing, in addition to the fact that the total cumulative burden of toxins taken into the body will be higher if started at a younger age.
Now, to address the question about breast cancer, I am not sure where she is getting her data from. Smoking has been proven over and over to cause cancer. Those with a family history of cancer are at an increased risk of cancer, as you know. What might not be as obvious is the fact that the risk of other cancers also increases when you have family members with a different type of cancer. A common example related to breast cancer includes ovarian cancer, which is often found in families with breast cancer.
There are a number of reasons not to smoke, and cancer is just one of them. Especially given your family history, I would recommend that your daughter see an adolescent medicine doctor
for further smoking cessation guidance.