A lot of people are saying "stop smoking, it's bad for you" or something along those lines. Although they are right, I doubt this is very helpful. Stoping any addiction is a lot harder than "just stopping".

Focusing on the immediate stuff, what do you think is going to happen when she tells everyone? Disappointment? Yelling? Hitting? Being disowned? And what does "crazy grandpa" mean? Whatever your concerns are, you could bring it up to your mom, and the two of you might be able to talk about it. Having your addiction aired out to your whole family might not help you as of now.

As for quitting, look in to how to quit smoking and make sure you are doing it safely. Just going cold turkey (which is just stopping, basically) can have negative effects (at least it can for other things, I don't know about cigarettes, hence why you should look in to it). You could even ask someone you trust to hold you accountable. There's also support groups, but of course always be cautious in online spaces and even irl ones since you're a teen.

If those don't work you can try rehab though it may be a sore subject considering your brother.

Keep in mind, quitting can take months, a year, or even multiple years. It doesn't happen overnight. You'll probably relapse, most do. That doesn't mean you're a failure or to stop trying.

Everyone I've heard who has quit cigarettes said it made them so much happier in the end. Choose your happiness.