Tell your c6 he can be a good son and his mother doesn't need an extra daughter. Being a good mother and a good wife is enough. Thunder god punish bad sons, never 'invisible' daughter-in-law.
Exclude yourself completely from c6 family functions and let them see bb in public places. Ask him if he wants peace or you going to see his family. Let c6 see his family, he wants to be a good son, you have no rights to stop this. But you have rights not to socialize with his family.
Be firm about the smoking issue, ask him if he would give adult medicine to bb, if not, why nicotine? You have every right to protect your baby and keep bb away from bad influence.
One more thing, a good son sees no wrongs in his family. He is blind. I don't comment on my hubby's family. What they do or say is not important to me.
It's about control & manupilation. Your c6 and his family are playing this game. Refuse their unreasonable demands and offer no explanation to his family. Explain to c6 (in private) in 3 sentences and leave the scene. No more arguments and no persuasion. Show c6 he can't change your mind. End of story and act normal, show him you are not bothered by the problem and continue your sweet sweet self to him.
Men don't want trouble. If you offer him a way out, he quickly takes it. If you don't, he goes to his mother.