Because of the action of the hormones in the oral contraceptive pills on hormone-receptive tissues in the body (which include breast tissue), there can be a wide range of side effects from taking the pills. Among these side effects, one of the most common is breast tenderness or enlargement. Most of the time, individuals will notice a "tender" sensation in the breast or a subjective sensation of breast enlargement. Rarely the breasts can be objectively enlarged and noticeable to others, such as yourself.
Whenever oral contraceptive pills are started, the person taking them should have regular followup with either their primary care doctor
or their OB GYN doctor. This followup is to monitor for side effects, both less serious ones like breast tenderness or enlargement, as well as rarer but more serious side effects. Sometimes, it may be necessary to switch to a different type of birth control
pill or to a different form of contraception altogether if the side effects are not manageable.
I would encourage your sister to follow up with her doctor if she has any questions or concerns about her oral contraceptive pills and the side effects they may be causing. I'm certain they will be happy to discuss the issue.