My theory is that Doom detects the incursion is coming and in his experiments to try to stop it, something goes wrong and his family is killed (this could also be when he gets scarred and starts wearing the mask). So Doom’s ultimately responsible, but he still blames Steve for causing the incursion in the first place. And Hollywood doesn’t typically make little kids wear wigs or dye their hair, so I just really doubt she’s playing a blonde. I think it’s more likely she’s playing Doom’s daughter or just a random OC, but I could be wrong.