You may be better penning her and the other twin, if she doesn't but the little one have him in there too (although there may be danger that she lies on him...), and milk her regularly, and once he gets stronger hold him to suck onto her.
Are his knee joints fused or can you straighten them up by massaging them? If the second he should come right, may have just not had enough space inside. Some kind of splint may help it. But if his joints are fused and he cannot walk I am not sure what his outlook is. Were yours vaccinated against Schmalllenberg?