My Matilda didn't sit till about 9 months.She was a roller,or she'd lay on her back playing.Like you say,she'd sit on someone's lap,but if I sat her on the floor,she's just fall down.
Maybe your little one's just not ready.Some babies are sitting before 6 months ,some not till long after.Don't worry she'll get there in her own time.
There's no set order for these things,they just do it when they 're ready.With the fact she's crawling,she'll be strengthening her muscles as she goes.The sitting ring thing probably helps too.