Hi,
I don't think that these pink spots on your sister's breasts have anything to do with breast cancer,especially when both breasts are involved..This problem can indicate a range of conditions.
The skin on the breast, like anywhere else on the body, is susceptible to rashes/irritations,allergic reaction to soap,lotions,and detergent she is using in washing her undergarment.There is also a possibility of a local irritation caused by the material of her brassiere or wearing improperly fitted ones.
If these spots enlarge and don't go away in a few days,the best thing to do is to have it checked by her Physician or a Dermatologist.
Best wishes to you both...