My daughter was very sturdy - good head control at 8 weeks and so on, and was sitting very confidently at six months. She was a contrary madam though and didn't want anything but milk until 14 months, although did deign to eat a few bits of food occasionally.
DS, on the other hand, is five months now and I'd say has only just got good head control in the last 2-3 weeks. I can't see that he'll be sitting unaided in a month's time, although I might be wrong. He's not ready for weaning yet in any case as he's still got the poky tongue thing. He can bring objects to his mouth but does tend to poke himself in the eye en-route to his mouth!
Is the sitting thing important? What about if they can sit up well but with support?