I would continue to do what you are doing, Occasionally bring the subject up. From a mom's point of view: maybe she is waiting to see if this idea "blows" over. The reason I say this is often times my kids will discuss things, and my way of finding out just how serious they are is to see if it "blows" over or does it remain a definite interest to them.
Based on things I have read in your posts, I believe you spend time with your dad? Have you made plans for how the chickens will be cared for during your visits to your dad's house? Maybe she is concerned she will have to care for the chickens while you are away?
I was also wondering, when you have pointed out the benefits have you mentioned that you can offset the feed costs some by feeding the household scraps? I know that many scraps can be added to the compost for disposal, but if you haven't already maybe you could point out to her that feeding the scraps also cuts down the household waste, that what otherwise might be thrown out gets fed to the birds that are in turn going to lay eggs for you to eat, so in theory what would be considered wasted food, in the end is not wasted at all.
Depending on how you think your mom would feel about chicks, maybe you could mention to her that bantams are very good setters and mothers, and that it would also be possible to let one set, so that you could sell the chicks so you would have less out of pocket expense for the feed
good luck with winning your mom over, sounds like your step dad is definitely interested, that could really be a good thing, maybe he could have a little "pull" with your mom if he wants them too, maybe, just maybe he will convince her.
I have some really pretty bantams, not sure the breeds but they are VERY small. I will see if I can get a pic, and post it if I can figure it out
I am hoping they will set on eggs. If that happens, and your mom agrees I would be willing to help you out with some chicks, if you are close enough to me
I am in Vermont.