For the past three weeks or so my DH and I have been fighting with SS over his math homework. My SS11 is High-function Autistic, and I understand that anything involving changing can be challenging to him. BUT, GOD, a wrong answer in math is a wrong answer, no arguing is going to make it right. My DH and I have both exhausted ourselves so much to a point my DH even said that he was seriously considering sending the son to his mom forever. (SS lives wtih us 100% of the time, last year he only spent time with his mom less than a month in total)

Oh boy, steppers.  I'm at a complete loss and need your wisdom.
SS13 is autistic.  We hired a nanny for the summer to watch him.  SD15 is also home, but she has no authority over her little brother, he doesn't listen to her at all.  It's also not fair for her to give up her summer to babysit her little brother.