This might be a good opportunity to address your dynamic head-on. When she starts complaining, you could go farther than "do you want to vent?" and say, "we've been talking about your struggles for awhile, and I've had some ideas about new ways to approach things. However I've stopped myself from sharing them because I feel like we're kind of stuck in an outdated dynamic where I'm the little sister, and I only "receive" advice. The truth is that I'm a grown woman with a lot of experience, and a valuable perspective to share. We've grown up, but in some ways our relationship hasn't. Are you interested in letting our relationship grow up too, I know I am." See what she says.