Mother in law help?

Me my husband & our 3 kids live with my parents we have for 3 yrs when our younger daughter dev. some medical problems since then i've had to quit work to be with her.We have a lot of conflict about this situation, his mom is always puting her 2 cents in ie: brings up the bible & turns it around to benifit her, says I don't care since i'm not helping financilly,(she knows why trust me I'm far from lazy & would love to have adult conversation again & a PAYCHECK!) blames my parents on why we don't have a home(shes not financailly stable but my family is) She tells him it's because he's black (well if my parents didn't love him he would not be here)(I'm white hes black our kids are mixed)If I say salt is white, she'll say it's red and he'll come back and says it's red maybe pink! how do i let her know i'm his wife not her and still respect her as his mom? I've tried in person and on phone & back down should i write a letter and correct her on her words both biblically & her "wisdom" ???