I agree here, indeed the 2+ to hit and wound and Rend -2 are probably necessary to make the attack any good at all. 1 attack and damage D3 against most enemies are by no means good.
The brother's sword doesn't look too bad, mainly because very early on the weapon will only be Damage 1 or 2, and if you can kill the Twins during one of these early turns you can stop it becoming too powerful. Any army that can shoot a load of missiles at them and/or deepstrike a powerful close combat unit within striking distance of them shouldn't have much of a problem with them. In addition, the mortal-wounds-based-upon-the-battle-round ability is a once-per-battle thing, which is fine, and the fact that you have to choose whether to use it early while the twins are still alive and deal a weaker blast, or save it for later in the battle to deal a stronger blast but risk the twins dying before they can use it gives the player a tactical decision to think about.
Of course, we need to wait until we see what other special rules these and other Lumineth characters have, and their points costs, before we can truly judge them, but certainly at the moment they don't look that scary.