Depending on where the tenderness is, it is probably just puberty. when you first start to develop breasts they are tender, and hurt really bad when someone hits them. As she develops more the tenderness will end naturally. If it doesn't stop then you should call a doctor
The breast does get harder and sore when it first begins to develop. Often girls who have read cancer-symptom lists think they have cancer, and all it is, is the breast buds beginning to develop. Don't let any doctor suggest removing them, believe it or not, it has happened, giving the young girl scarred breast development later.
Do you mean the whole breast or the nipple area , at 12 year old she would be going into puberty and changes to the breast are normal , what does the step mom think is happening ? I would definatly have her checked by the doctor if there is some concern