Yeah, I wouldn't necessarily read anything negative into the fact that she hasn't sent you the instrument yet--more like she just hadn't thought it all through. I know lots of people like this (my relatives). "Oh yeah, I'll get that to you one of these days." Only it's a multi-step process--first have to get Thing out. Then let it rest for a while. Oh, looks like Thing needs to be repaired. Let it rest for a while. Take Thing to repair shop. Find out it will be there for weeks. Get Thing back. Let it rest. Slowly realize that shipping Thing is a hassle, thus prolonging the time Thing rests. Etc.. Meanwhile, the need Thing was going to fulfill has come and gone, the child has grown out of it, I went and bought myself one so I could keep going, whatever. It's not malicious, it's just sort of... on their own time frame, without really thinking about the other person's viewpoint.
I would get another Thing for DS and just not worry about it anymore. That way if this one ever arrives, you can be pleased and find a use for it as a backup or whatever, instead of getting stressed and bugging her. Bugging doesn't seem to work with people like this, at least not the ones I know. Bugging just attaches more guilt to the Thing, so they're less
likely to go near it in the future. Acting like it's a great idea, but you don't really care if you ever get it, seems to speed things along for some reason. (Or it just makes it seem
faster, because you're really not worried about it.) It's... weird.