hi S., and congrats! i dont know all the details, but i would say that your sister is absolutely right to be concerned. i would get that baby off the bottle right now if at all possible. and if there is a real medical reason that the baby must be supplemented, be sure that she nurses the baby first every time, then offer the bottle after the baby has emptied both breasts. she must nurse or pump at least every 3 hours, especially at the beginning, this is the crucial time. i dont know many people who supplemented in the beginning and were still nursing a month later. her milk supply will drop very quickly, and the baby definitely may develop a preference for the bottle, it is much easier for him. he needs to develop his sucking at the breast. a big baby is not a reason in itself to supplement, if anything it is a reason not to, as she needs to stimulate her breasts to make more milk. people nurse bigger babies than that, they nurse twins and triplets, etc. they nurse toddlers and newborns at the same time. my first baby was almost 9 lbs, the nurses in the hospital convinced me that giving him bottles would be no problem, and it lead to 2 months of a nightmare getting him off the bottles, it was horrible, crying and bleeding etc. my daughter was over 9 lbs, and she was in the nicu and had to have bottles, they didnt let me nurse her at all, so i pumped every 2-3 hours around the clock for almost a week, then she switched to the breast with no problem at all. my last son was the biggest yet, at 9.5, and he is the only one that has never had a drop of formula, and he had totally different problems in the beginning, a poor latch, which we also overcame.... my point is that all babies are different and all nursing relationships are different. if your sister is truly determined to nurse as i was, she can overcome it like i did with my first, but i am still so angry that my first 2 months as a mommy was spent mostly crying and stressing, and it was so unneccesary, just due to ill-informed, careless nurses. ill never get that time back. best of luck to her, if you need more help please feel free to contact me, i overcame lots of different nursing issues over the years, and got lots of wonderful help from lots of wonderful people, friends and professionals, and message boards! good luck, D.