I have two brothers, and we talk when I call them to check in or if they need something (traffic on their drive, help with a project) or, for one of them, if he's killing time while driving. That brother probably calls me once every 3 weeks or a month. The other brother hasn't called me in probably 2-3 months. I used to call them, but I left so many voicemails without a return call, that I stopped trying eventually. Now I might try to call each of them once every 3 months or so. I get emails from one brother about twice or three times per week...they're usually some photographs he's taken and it's a group email to several people in the family. The other brother does the same kind of email maybe once or twice a month.
They're really close with each other, though - I think they talk several times a week. They fly back and forth to visit each other maybe 6 times a year. One brother hasn't visited me ever (I visit him once a year or so) and the other brother visited me once for 3-4 days when my daughter was born 9 years ago and once for an evening about 3-4 years ago. I've been to visit him only once.
On my husband's side of the family, though, there are 4 sisters. I'm in much closer contact with 3 of the them than my own brothers. I talk with two of them once a month or so (out of genuine interest to talk or make plans), and the other every other week or so. While I love his sisters, I avoid talking to all of them (along with his folks and anyone in my family) when I'm depressed. If someone happens to call me, I just can't bring myself to do much more than answer questions and beg off the call with some excuse or other. Often I just don't answer the phone at all. I just don't have it in me to fake how I'm doing for very long, and I'm not interested in opening up when I'm depressed. If someone gets me to really start talking about how I am, it's awful for me. I end up crying and with a headache and brain fog for the rest of the day or several days. I stick to generic conversation topics - what's been going since we talked last, upcoming plans (if any), how the kids are, and sometimes even literally the weather.
If you're wanting to be really invested in touching base with her without setting her off (sorry if I'm not wording that quite right), maybe try to come up with some neutral topics ahead of time. You both like music, it sounds like, so maybe check out some new songs that you'd like to share with her. Think about a book you recently read or a movie you saw that you might want to bring up. A new restaurant you tried, some recipe you're loving right now, any hobby you're into & what's going on with that. There have to be things in your life that aren't your boyfriend to talk about. Also, it's much easier on me when I'm feeling down if people say something like, "Just wanted to call to touch base/check in with you...we haven't talked in awhile." or something like that, instead of asking how I am and trying to get me to talk about that.
Don't know if any of this is helpful to you or not. I do know that it really sucks to feel like you don't have the relationship with your sibling that you want or envisioned. Or that you're more invested than they are. Or feeling let down after you talk with them. No fun at all...
**Sorry to hijack the thread a bit & write in so much...I have found I'm nearly incapable of not writing everything that comes into my mind!