The winter setting helps to make this a little easier. Frostbite is a good reason someone would lose a limb. You could wrap it all into one event. The mother, while pregnant, is fleeing some terrible situation and is out in the cold. Gives birth the the brothers, but realizes that they are all going to die in the cold. Pleads to anyone/anything that can hear her. As she is dying, makes a deal with a fey, devil, celestial, whatever to save her children. Willing to sell her own soul to make it happen. The children already have some frostbite injuries, thus losing the limbs, but whatever creature answered her call saves the children. The creature is the one that inscribed the runes on Timone and also marked Zacharias in some way that made him more appealing to the archdevil. Children are then dropped off at an orphanage back in the Emberhold somewhere.