I don't really remember when .Alyvia started sitting up (isn't that awful?), but I do know that we put her in her bumbo a
lot. I don't know if that helped her to learn or what, but it didn't take her too terribly long to master it. Her biggest problem was that she loved to "stand" (supported) in laps, so anytime her feet were touching a surface she would instinctively straighten her legs. So when we were trying to help her sit, she would tighten her whole body and her head kept flying straight backwards towards the ground.
But from the sound of it, .Amelia's getting close! In a week or two she'll probably have it down. Babies are super clever, and once they figure something out they're good to go. You'll have to take some pictures for us!
One thing I would do is try to keep a pillow behind her just in case she falls backward so she doesn't bump her head. Other than that, just try practicing with her every day sitting her up and handing her a toy or something.
Alyvia was sitting long before she was crawling, but she did the same thing on her tummy that .Amelia's doing. Eventually she started working her way across the floor towards her toys, and did this half-crawl half-drag thing for weeks before she discovered that she has knees and feet. I want to say she started that around 8 months, but my memory is horrible so don't quote me on that.