My baby is 22 weeks old. She can now sit up unassisted, easily, for 10 minutes at a time. She'll eventually tip over, but she's very good at correcting herself if she starts to tip slightly one way. She also bends over to pick up toys from the ground while she's sitting.
She also ALWAYS wants to stand. If I try to sit her on my lap, she just stiffens her legs and pushes off me wanting to be held in a standing position. She sometimes refuses to sit, preferring to stiffen until you hold her up. She has been doing this for a long time (since bearing weight on her legs around 12 weeks, maybe). She is getting quite good at standing, too, and really doesn't need much support. She's definitely going to be an early walker.
However, she is not even really close to rolling over. She hates being on her tummy and will usually just push her head straight down and start crying. We still do tummy time for a few minutes each day, but she doesn't tolerate it well. I really thought rolling over would be an earlier milestone than the other things. Anyone's five-month-olds not rolling over, but are sitting up? Strange kiddo...
Re: Sitting and standing before rolling over?
We haven't worked on standing yet, but DD mastered sitting unassisted about 3-4 weeks before finally rolling this week. She'll eventually get there.