Very upset with my husbands mother i dont consider her mother in law

She recently is claiming my child isnt her sons baby !! She says my child isnt light that he is white! Remind you my child is bi racial and his father is very light skinned ! Were married ive never committed adultery let alone have a another option or chance on having a different father for our child ! I told him last night shes ruin N our marriage ans making me feel like im being pushed in a corner constantly talking about me to everyone !! Im not going to deal with this crap for a life time !!! What should i do ? Also im doing a dna test to shut her up and shut her out of our life knowing hes his and waste N time and money and beinf talked about as if im a whore im done wirh her she disrespected my marraige me and my child