MIL &SIL turning step son against me 😪

Ok so my MIL and SIL have never liked me (they don't no a damn thing about me after 4and a half years!) but watever I've grown used to it my step son lives full time with me and my husband (we do not live close to his family ... ) so Billy (his son) doesn't see them much so one weekend he went over there and when he came back he was COMPLETELY different toward me he was like another person we'll eventually it comes out my MIL and SIL was talking shit about me to HIM ... it's been 3 weeks since all this happened I tried to talk to my MIL and SIL but it only made things worse and they don't see anything wrong with ruining my relationship with my step son and my step son is still treating me different and acting like I don't exist.... i don't know what to do 😪 we had such a good relationship! 
Sorry for the long msg just need help.