Simply put, Weapons are genetically predisposed to developing powers that transform all or part of their bodies into a particular weapon.

In the case of the Thompson sisters, they transform entirely into a pair of precisely identical handguns. Due to the fact that they are related, their souls resonate easily, and they are therefore able to wield each other.

Otherwise, they might become impossibly heavy, or burn the wielder. At any rate, attempts to use a Weapon with whom one is incompatible risks harm to both the Meister and Weapon.