First off, your dad is responsible for his own health, not you. He is the one making the choice to smoke. He has free will and he needs to use it.
That being said, if you feel that telling him your brother's sins is only going to continually upset him, you have to decide what good will come of it.
Will your dad properly discipline your brother so that he corrects his sinful ways? Or is he a poor disciplinarian who is too weak to chastise his children? I'm not trying to get a rise out of you; it is a legitimate question and your answer should largely influence how you will act.
You can promise your brother that you will not tell your dad everything - so long as you are not put into a situation where you would sin. If your brother is committing sins and you know about it, you still have to oppose him, either by entreaty, admonition or rebuke, if you know about the sin, otherwise you are giving tacit consent to what he does.
Depending on how much influence you have on your brother, he may amend his ways, especially when you point out to him that it is not you telling your dad that is influencing him to smoke, but your brother's sins that are driving him crazy, and that your dad would find out about them one way or another.
Your step-mother needs to wise up and realize this too. You don't protect the sinner by hiding his sins or by ignoring them o their effects - that doesn't help him at all. The sinner needs to be corrected by SOMEONE, your dad would be the best person to do it - if he were Catholic - but since he is not, and since your brother is not either (I presume), and since you are an catechumen, I believe that it would be best coming from you.
Express to him the truth that those who willingly and delightfully sin will be delivered up further and further to sin and infidelity and blindness and can end up in no other place than hell. Preach unto him Jesus crucified, then the two of you get all your doctrine straight, make sure you aren't communing spiritually with any heretics, schismatics or Muslims, and get yourself and your brother baptized, heck get the whole family converted unto the God of salvation.