I would tell her what I tell my 7 yr old when she's upset she's left out of something that dd1 (10) gets to do. There have been several things lately that are 8 or 9 or 10 and up. It's not my rule, but I can't break it for dd2 either. What I tell dd2 is that when dd1 was 7 she didn't get to do that either and when dd2 is 10 she will have a chance to. It seems so simple, but sounds like your sis is just not getting it. When your 2 yr old is 9 she'll have a chance to take the classes, etc, then.
I'm the youngest and I so know about the childless older sibling thing. My brother and I are actually semi-estranged over some cr@p he threw at me when my kids were little. My older sister (who has kids) is a little more understanding although her kids were pretty easy and my dd1 has always been pretty high needs. Sis is still a bit bossy, though. It's just her personality. We're really getting along better now than we ever did as kids, though.