I think it’s to be expected when you have two siblings who are already bonded that it will take a while before they look to you rather than each other for affection. You’ve got some great advice above from KJ. But also bear in mind that most young kittens don’t want to be picked up for more than a second — there’s playing to be done! — and many cats never like being picked up (in fact, most). If you think about it, being picked up from above is hard wired as a danger signal — in the wild, it would be bigger predators (hawks and other animals) that would be doing it.
I think time will solve the problem, if you are around them when they are playing then you might find that a played-out kitten will choose your lap to crash on, and the other one might come and join them! Especially if you are sitting on the floor on their level and just happen to have fingers that have been dipped into some plain yoghurt or the odd yummy treat to give them. You could also try having a little one-on-one time with each one, playing and interacting. Every kitten is different and, as you say, you can read all the theory in the world, but in the end, your kittens will end up teaching you what they like. If all the grooming you can do right now is a quick ‘drive-by’ swoop with a comb or brush as they rush by, that’s fine for now, just get them used to it. If they collapse in your lap, pet them and try a a little grooming then, but just a little and often. If they really are still impossible to pick up or handle in a few weeks, when you are both feeling more comfortable with each other, then come back to us (or talk to your vet), we’ll talk about scruffing 🙂 One good thing to do is to get them used to their vet carrier by leaving it out so they can play in it and around it and get to know it without it being ‘that scary cage that only comes out when we get trapped and taken to a place where we get prodded and injected!’ Then when you do need to use it to take them to the vet, they won’t be terrified of it.