My dd had a hard time with bottles.
She wouldn't take them from me, and wouldn't take them if she thought I was there, or going to be there very soon, either.
We tried a million different kinds, found 1 she tolerated, and experiemented with feeding positions, locations, and people.
Can your sister visit her baby on her lunch hour? I did this, and it went a long ways towards how well B did, those first few days. At least I knew she wasn't going to go longer than 4 hours w/out. And, of course, she did nurse more at night to make up for it. Which was wearying on me, but seemed to suit her just fine. She turns 2 on Monday and is still nursing.