However, I think that he showed restraint; given that hamsters can crack open nuts with their teeth, I'm guessing that he could have left dents in my bone, but he didn't even break the skin. In other words, he just wanted to send a message of "Oi, watch it!" rather than actually trying to hurt me.
I'm using a Savic "Mickey Max" cage, which has 5mm gaps between the bars; since they're dwarf hamsters, they might be able to squeeze through a 1cm gap.
Still, it's nice to see that sibling behaviour doesn't change much between species.
However, I noticed that the hamsters sometimes put their legs between the bars when they were walking around, and we were concerned about injury, so we put down some newspaper to cover it.
The side effect of this is that if one of them stops then she gets spun around and then flung off; I've seen them do a complete loop that way. At first I thought that they were just a bit dense, and hadn't realised that they need to slow down gradually, but I now wonder whether it's deliberate (like a kid jumping off a swing at the high point).