My daughter was a "lazy nurser", she would sleep like a charm while trying to nurse.. it would take about an hour to get her to feed, and she wouldn't sleep a wink not on the breast. Her pediatrician. was always concerned about her weight gain, she wasn't gaining as fast as she should have been, so she told me to feed her formula but to still offer the breast. (my breasts did not like pumping, and I tried many models, most I could ever get was 1 oz and that was out of both breasts) So I gave her formula. I would try to nurse her exclusively for at least 3 feedings and then for her other feedings I would offer the breast for 5-10 minutes and if she fed great if not I gave her a bottle, she needed the calories.
As for the whole formula vs breast milk debate.... Breast milk is BEST but formula is FINE, (and a tiered, overly stressed mother is BAD). I had a friend of mine who is a nutritionist explain it to me like this... Breast milk is like eating ONLY all natural raw foods, which is so much healthier because it hasn't been processed and is more bio-available to the body, whereas formula is like eating only casseroles, boxed cereal and power bars... it has all the nutrition… all the parts are there but they are just not in the "best" form for the body to make the best use of. And we all eat cooked foods, and processed foods. So I say unless YOU only eat all natural raw foods and absolutely no processed foods of any kind including vitamins (which are processed) then don't be too hard on your sister. The breast is best there is no doubt, but feeding formula will not harm her son, the human body is an amazing thing.
So please don't belittle your sister for "resorting" to formula, I know firsthand how frustrating breast feeding a "lazy nurser" can be, and I think it is only because I was not made to feel guilty about it that I am still nursing my daughter today (she is 18 months), but having a new born baby is hard enough and if you push her too hard to breast feed she may just give up all together. Encourage her don't discourage her by pushing too hard. I think an all or nothing mentality is not healthy. My daughter never stopped breastfeeding, I think because I always offered it to her and it was such a nice bonding moment, but there were defiantly times when she preferred the bottle, and that was OK too she was still getting nutrition.
I think being given the “permission” to offer formula helped with breastfeeding because it took the stress away from it, and that’s why I was able to continue breastfeeding. So again don't push... encourage and support.