I agree, step mum seems unhappy, and takes it out on everyone. At least you can take some comfort that at least its not just you! Have you ever tried talking to her about it, but not after an argument or anything? Or next time saying something like 'I didn't like how that comment made me feel' or something a bit more eloquent than that.

As for your female relatives, am sure they dont really believe what she says. If she's as mean to everyone as you say, I bet they know she's just stirring and gossiping. And if they do believe it, then let them! More fool them.

Life has taught me that people like that rarely change, and perhaps if possible you could learn to ignore her and her comments, keep any exchanges to the bare minimum. Sometimes people stop if they dont get a rise out of the person they're being catty towards. As for the wedding, you're going to look all lovely, hair & make up etc, and am sure will feel a million bucks! Try putting a big smile on your face for all to see and dont let her or any gossip drag you down and certainly dont show it & give her the satisfaction. I bet it will be a lovely day, seeing your brother getting hitched and am sure she wont spoil it.