She didn't set the house on fire that her mom died in and her mom is not dead. She set a barn on fire when her mutant powers manifested. And anti-Romani/anti-mutant bigots set their house on fire. It was believed that Django and Marya died in that fire. But later on both were shown to be alive. Wanda's house that was burned by anti-mutant/anti-synthezoid bigots was the one she shared with Vision. Also her mom was changed, again. So now it's Natalya and not Marya, nor Magda.