If she is daring enough to brag when she's been "bad," then the old wash-the-mouth-with-soap idea may just backfire on you; she may "get her back up" if you do that and be even more defiant. Punishments can actually be seen by kids, believe it or not, as a form of attention, and she definitely wants your attention, even if it's the negative attention of time outs and fussing at her. She curses, she gets attention, and her little siblings don't.
I'd suggest immediately and unequivocally withdrawing ALL adult attention when she curses. Time outs can do that only if you do not talk to her, engage her, fuss at her or give her any form of attention while she is in time out, and also if she stays in her time out seat and doesn't defy you by getting up, getting a toy or book, etc. If you have to restart time out 10 times over two hours just to get four completely quiet and still minutes of time out (followed by her telling you what she did that got her in time out and an apology to you) -- then put her back in time out 10 times even if it takes hours! TV's "Supernanny" has that right -- time out won't work if the child moves around or if the parent talks to or in any way engages the child during that time, and time out should restart if the child gets up. Maybe she's not doing that but it's a thought, if she is.
Alternatively, you can say very coolly (not fussing at her), "Stop right there. You cursed. I cannot talk to you when you use those words because they are not words I accept or recognize. When you can apologize and use real words we can talk again." Then walk away, start your own chore etc. and don't engage her, answer her, etc. until she apologizes. Sounds cold, yes, but lets her know this is NOT acceptable behavior and won't be rewarded with your attention. Then thank her and give her attention once she apologizes, and drop the subject.
And be sure to give her lots and lots and lots of praise for good behaviors. If you use a reward chart, focus it not on "not doing bad stuff" (It's hard for kids to understand a negative like that) but on "doing specific good behaviors" like picking up toys, washing two dishes a day or any other chore that helps her feel empowered and like a big girl who gets mom's attention by being good, not by being naughty.