breastfeeding and work, help!

So I've been exclusively breastfeeding since my baby was born. She is now 3 months and I will be returning to work in about 2 weeks. I've been pumping and trying to introduce her to the bottle. For the most part she takes the bottle ok but there are times when she refuses it. I'm trying little by little to give her the bottle but I'm so stressed my sister won't be able to feed her while I work? Should I be so worried? Any advice? I've tried about 4 different bottles and she takes them all the same. None she really likes.