Glad you are feeling a bit better about it already.
My daughter sat unaided at just under 6 months (for short periods anyway) but my son is 5.5 months and no way will he do it! He will be another month at least I am sure, he tilts badly even if I am fully supporting him. I would not be surprised if he isn't sitting unaided by 7.5 months because he really is not practising or interested in that skill right now. Then again, my daughter didn't even roll over until about 10 months (or crawl until 12 months) and my son can roll and is desperate to crawl.
I haven't parented them any differently other than just following their leads. My son likes to be on his tummy and spends a lot of time there, my daughter hated it and would not spend a single minute on her tummy until 10 months or older. So I just did what suited them.
If I didn't hold my daughter sitting up so much when she was a baby, maybe she would have crawled sooner, who knows, but then again she'd also have done a lot more crying because it's definitely not what she wanted. Same with my son, maybe if I held him upright more he'd be closer to sitting (he's only 5.5 months but noticeably less skilled at sitting with support than the other babies I know his age, he requires full support not just help balancing) but he's happier on his tummy so I just let him get on with it.
I think you just naturally do what the child prefers. You don't even need to think about it. When the baby cries or writhes, you pick the baby up and try something else etc. That's their only real way to communicate and I highly highly doubt you are leaving your twins in positions they dont enjoy (you sound like a really dedicated mum in all your posts) so from that I already know you're doing everything you need to to encourage their development. I really think the thing to do is just follow their cures and they'll let you know what order/speed they want to learn stuff.
When my daughter was 10 months she went to the paediatrician due to slow growth, and she said to me about development exactly what Larkspur has put. When I said Joni wasn't walking or weight bearing yet (she couldn't weight bear until 14.5 months) she simply asked, "but is she getting better muscle tone over time and improving her posture, movements, coordination etc". She was only interested in progression, not ticking off a single specific milestone.
It's definitely easier when they can sit up. I'm sure with twins it will be doubly so because you have to be forever sorting two people. When they can sit it's so much easier to feed them and they can get to their toys from a better angle. I can totally understand why you are getting frustrated, don't think I'm saying I can't! But I definitely don't think you need to worry or stress or think you are getting something wrong. They are happy on their tummies and you are simply following their lead letting them play however they feel comfiest. Xx