It sounds like what you're really upset about is your stepmother sticking her nose in it. As a stepmom myself I stay way the hell out of "family business", KWIM? I certainly have opinions but I keep them to myself, lol.
You must decide to be at peace with your decision, make a trip as soon as possible to have a special one-on-one birthday visit with your grandparent, and mentally tell your stepmother to go to hell. She is obviously trying to make you feel bad--why give her that power?
I'm the wild child in my family, too, and occasionally have scheduling problems that prevent me from attending family events. I am the only one who A) ever moved away from the home town, B) works odd hours (7PM -7AM) including weekends and holidays, C) has to plan my work schedule 2 months in advance, and D) never had children (gasp! I KNOW!). I've decided that everyone else just has to deal with it. I am almost 50 for crying out loud.
Just out of curiosity, is the handgun class something you have to do for your job, or just for yourself?