Hello,
First and foremost I would advise you to isolate yourself from others.
Secondly, get tested for COVID19 by RTPCR swab test.
For breastfeeding mother, i would like to know if she is having any symptoms and other details.
Next Steps
Consider an online consultation to get specific assessment and treatment for your sister.
As she is a breast feeding mother, it is safer to treat her according to her symptoms.