How do i stop his sister saying telling my son she is his betend mammy?

i've split from my ex because i couldn't put up with his family anymore firstly we went out together for a year & 1/2 his family didn't bother with him , but when i was pregnant unplanned his sister who is mad to have a baby told him it was easy to get rid of baby &so it wouldn't upset his parents& it didnt suit her he told me this ,since child was born his father constantly put us down, not feeding child right , house not clean enough ex never stood up to them they bully their opinion ring on holidays to tell us what to do, & his sister has since told my son she is his mammy . i've told my ex this & he says child made it up ,child was upset over this he is 4 & wouldnt make it up & ive been left very stressed by this how do i get by this ? oh ex sees no wrong in his family they could insult child & he would blame me for it whats best way to deal with this rationally