Dear Andrea,
I guess I am just a cynic at heart. I know there are wonderful Doctors out there- but not all are astute as we would like to think. You only have her report regarding her last checkup with the Physician- all information conveyed to the Dr. or from the Dr. is confidential, unless she chooses to honestly share what was said with you.
If the Dr. is in touch he/she is monitoring the infant closely (hopefully) and will pick up on her infant's "Failure to Thrive" but that is supposition- IF her Dr. is paying attention. Lots of babies don't do very well initially for all sorts of reasons. Her Physician may be waiting to see how the baby does before trying to intervene.
I can understand completely your concern- you are correct in your observation. Baby's need nourishment often, besides the physical aspect of comfort and nurturing.
I like the advice to give your sister-in-law information that may clear up her misunderstanding. There are many Lactation Consultants available, plus books on nursing, however, if you feel the infant is in danger and she won't listen and is not in tune with her infant's needs- it could become life threatening. If you feel that it is that serious, call her Physician. Alert him/her to your concern and make it very clear that you want the information conveyed anonymously. Remember that the Dr. cannot even disclose to you that he is seeing the baby. Try to relay facts, times, dates etc. Actual quotes from the mom will help the Dr. understand more clearly.
Prayerfully approach your decision, it's worth a baby's life.
It is painful to know of something so serious and not try to do what you can. Education of the mother is, of course, the best course of action.
I really feel for you. It is difficult to know something and let it go, but after you have tried to do what you can, you have no other recourse than "let go and Let God".
Remember them in your prayers.
Best to you,
H. B.