Is smoking the same cigarette with her husband's cousin considered normal behavior, especially given her previous ill-intentioned actions towards him?

If it is confirmed that your husband is still smoking with his cousin in the manner described, we advise you to be patient with him and continue to advise him and remind him of the impermissibility of smoking, and that this girl is a non-mahram to him, and it is not permissible to interact with her except according to the Sharia guidelines. You must work to turn his heart away from her by treating him well and adorning yourself for him. The girl must be advised and threatened with informing her father if she does not stop these actions. As for your in-laws' statement that this behavior is normal, it is incorrect and involves condoning evil. They must repent and work to deter their daughter.