I'd chalk it up to personality difference. Just like some adults are not baby people (like myself, for instance, unless they're my own babies), some kids aren't baby crazy. Especially when they're so close together.
For my daughter, she didn't mind the babies but she didn't go gaga for them either. In fact, after they were home for two weeks, they were ho-hum and might as well have always been there. (She was 17 months when they were born, I suspect young-toddler sense of time/history factors in there)
She's still not interested in babies in general very much (there have been 2 I can think of who have captured her attention, but that was on an individual level, and they weren't her brothers). That's cool with me. Her brothers, however, do like babies and go nuts over them. (maybe because they've never lived with one younger than them?
) But I can see why in their personalities they'd be more into that.
Sometimes siblings aren't best friends, sometimes kids don't especially love babies. It's just a personality thing. And it could be a personality dynamic thing too...where two sibs who are especially close are that way because of a really good personality fit, or find it hard to get along because of very divergent (or too similar) personalities.
And keep in mind that the girls you had over may very well have reacted differently towards a baby that they had to live with 24/7, as opposed to just visiting.