J.M.
When I decided to introduce the bottle at 5mths my son absolutely refused, at first. I hadn't had this problem with his big brother so I was very perplexed. Several tries, several types of nipples and bottles, and all he would do was chew on the nipple, he wouldn't latch on. First, be sure its the bottle she refuses not the contents. this applies if the bottle contains formula instead of breastmilk. Of ocourse the reverse could be true too. My son refused breastmilk in the bottle but finally took formula. For breastmilk he prefers it straight from the source. Also, I discovered that temp. was a factor at least for my son. He likes his bottles much warmer than I was heating them or that #1 ever wanted. This is true for a lot of breastfed babies. Breastmilk is usually body temp. (not room temp.) which can feel pretty warm to us. So Your sister may want to play with different temps for the bottle and see if that makes a difference.
When I was trying to get my son to latch on to the bottle I finally had to trick him. I sat with him like I was going to nurse with breast exposed and everything. I held the bottle up lining its nipple up with my own and kind of pressing the two together the best I could. When my son opened his mouth to latch on to me he actually got both in his mouth. Once he started sucking I eased my own nipple out. With the two in his mouth he couldn't actually get a seal around either to really latch on. It took him several seconds of active sucking on the bottle before he realized the substitution. When He let go of the bottle I repeated the trick. After about three tries he drunk about half the bottle and then refused to opened again. I did this at one feeding each day for several days. Eventually he would finish the bottle and then started taking it without me having to trick him. I guess once the feel on the rubber nipple was familiar to him.
Good Luck!