Overstepping boundries

At the end of the day it is YOUR wedding, not theirs, and if you think her personality won't fit, I think sticking to your guns and maybe even explaining it to step mom as such may make some head way? Some times knowing the reasoning breathes new air into the situation. OR have a family sit down with her and your dad (SIL too if you're brave) and deciding as a family how to move forward.

End the end, it is your day and it is ONE day.

If you want her as a bridesmaid or flower maiden, who cares what your stepmom thinks. I think either would be fine. Married couples are in bridal parties all the time. Your stepmom may be helping to pay but that is not her choice to make. I say do what you want.

I'de keep her as bm if that's what you really wanted her to be.

Please keep her in the role you wish.

that's very kind and mature of you to consider your sm's feelings since they are helping but they are not underwriting the whole thing and besides, these roles don't fall under the auspices of who pays.

maybe, nicely, ask stepmom if she had her wedding the way she wanted. yes? well so do you. no? well, maybe she would have preferred it and so do you.

what's important is to be calm and respectful, even when she isn't.

it will be great! come back and tell us how it goes/is going

Well I managed to convince my step mom to keep her as the flower maiden.

I remembered that I used my SIL's foot as an excuse to get her out of being a bridesmaid (she's had this foot issue for a couple of months that she doesn't want to sort out because walking on the crutch gets her a lot of attention and she doesn't have to do housework). So to get her out of being a bridesmaid, I was all like, "oh I don't want to cause more damage to your foot" and she lapped up the attention and consideration. So I said to my step mom that I can't have her now as a hostess/ usher/ greeter because that is just as much standing, if not more, and they may think that if she can stand to be a hostess, she can stand to be a bridesmaid.

Your first mistake was in removing your sister's wife (SIL, to make is easier) from the bridal party. That sent a message to your step-mother that this was negotiable. She put you in the unenviable position of having to lie to your SIL. While getting her back into the role of flower maiden is nice, I'd put her back where I originally wanted her.

When parents help pay for weddings, I do believe it does give them some influence in the choices that are made, but those choices should be relegated to things, not people (and least of all, your personal attendants).