I'm serious about the pipes. But here's my even more serious advice (and I've played the Irish pipes since 2004 -- frequently in public at Irish traditional music sessions and the odd gig -- and faffed around with assorted other things before that). Let her choose her music. Don't get her more instruments just for the sake of it, or push her to perform or whatever. She's 15. She'll figure out what she wants to do in her own time. But she'll need space to work that out. Child prodigies are arguably playing one or two instruments at a professional level by then, but the rest of us are puntering along, trying to find the music and the instrument that calls to us, and there's no rush. But it sounds as though she plays quite a lot of instruments, and she may be talented, but to become solidly proficient at any one instrument, most of us mortals need to dedicate a gazillion hours to that instrument, and it doesn't really pay off to spread yourself too thin amongst too many instruments. That said, some people somehow manage to do it well, many more do it poorly, and some instruments have similar-ish skillsets (guitar/ukelele/banjo/mandolin/bouzouki, or flute/pipes/tin whistle, or violin/viola/cello...), i.e. I could go from pipes to flute with less work and vexation than pipes to fiddle.
But really, it doesn't matter. Encourage her to play what she wants. If she really wants to try a new instrument, a fiddle, or an accordion, or a clarinet, or a set of bagpipes, get her one by all means, although many of those are not cheap. If she wants to put her music out there in public, definitely encourage and support her, but don't push her if she's not keen (I don't like solo gigs very much -- give me ensemble playing any day). Compliments from random strangers on the internet are nice, but if she doesn't feel like solo performing is her thing yet, that's totally fine. If she prefers being part of the ensemble in the band, there's no shame in that (and more money if you become professional, as few people make it as solo artists but lots get by as session/ensemble players). Maybe instead of getting her random instruments, expose her to different types of music. Irish and Scottish trad music, Breton music, Asturian music, Klezmer, jazz, blues, classical, non-Western music like Indian, African, indigenous Australian, Middle Eastern.... And modern fusion of any and all of the above.
There are 'folk music of the world' CDs you can buy that have all sorts of stuff. Putamayo is a record label worth looking into. I have zero connection to Ireland and Scotland (other than living in Scotland now as an American expat) but my Dad loved his world folk music and had all these albums lying around, and for as long as I can remember, I was drawn to Irish music and the pipes in particular. It was years before I got the connections to get a hold of a set and learn to play the damned things. Still, my parents pushed me and my brother to learn some sort of music when we were wee kids, and I messed around -- with no great degree of competency -- on the French horn, high school choirs, the mountain dulcimer, the recorder, Native American flutes, the harp, the bodhran (Irish drum), the tin whistle, before I finally got a set of uilleann pipes at age 21. My brother played around with the trumpet when he was about 8, and then settled on the didjeridoo. To be honest, the breadth of music my parents exposed us to, with all their weird and crazy CDs, probably had more of a profound effect than French horn lessons when I was 9.
So my advice? Get her a world folk music CD or five with music from all over the world. And a tin whistle. They're like seven bucks.