Your sister is suffering from spontaneous onset breast cancer. Though breast cancer is known to have a familial predisposition, majority of them are spontaneous in origin.
Your sister's case qualifies as a T1N1M0 tumour and whatever has been planned is perfect. The chances of recurrence following a Breast Conservation Treatment (BCT) is same vis–a-vis a more radical surgery.
You have not commented on the histopathology report including ER (Estrogen Receptor), PR (Progrsterone Receptor) and the Her2Neu status and the chemotherapy regime being given. This information is required to comment further on the adequacy of the treatment.
Nonetheless the broad plan being instituted in absolutely perfect.