Im about to have my 1st child, my sister feels she is being replaced?!?

Yesterday a bundle of baby stuff arrived from online buying, after these arrived she started crying to my mother, saying she feels she is being replaced... she is 17 almost 18! I dont get it? I get she is a little jealous all this stuff is being bought for the baby and not for her, but shouldnt she be happy for me? I can see why she would be getting sick of all the baby talk, it's like word vomit these days haha, but she isnt a very maternal girl, she doesent ooh and aah over babies like most women, she told me she is scared to hold babies etc and she just doesent know how to interact with them, I thought seeing as though my daughter will be a big part of her life she would love her, but now it seems she just doesent care and wants to be the "baby" still?? she has her own life, boyfriend and friends .. I just dont get it.. why would she feel like this?!?!?! we've always been really close and I want it to stay that way! thanks! :)