tough one, LMo. I offended my mother-in-law one time and it took years for her to barely speak to me again. It is still a bit chilly 15 years later. If you offer a heartfelt apology and it falls flat, then there really isn't much you can do but treat everyone nicely and wait for the ice to melt. Hopefully it will be days or weeks, not longer.

Maybe you could write her a letter? You are very good with words. If she reads it, there might be a better chance of her hearing you out than when you are verbal and she can shut you up.

What about your step-brother? Is he rational on this? Could he try to present both sides to his mom?

Wherever you go, there you are