OOops sounds like a little sibling rivalry to me.
What I would do is give your DD the choice of wearing pull ups or her pants in the morning. saying - If you want to be a baby like DS and piddle and poop on the floor and not on the toilet then you wear pull ups, if you want to do your poops and piddles in the toilet like a big girl then you can wear the pants with the hearts on them or whatever, and get a star on your chart / or a chocolate button / jelly tot or whatever you're bribe of choice is (I used bribes even though I swore I wouldn't when toilet training my DD)(they worked for the final push to completely dry, so to speak).
That way your DD gets to chose how she is going to be, and knows that if she piddles or poops on the floor, then she chooses to do this, and that she gets put in baby nappies because of her choice: So she's in control of something - having a sibling can rock their little worlds.
It is hard not to be annoyed or despairing, but if you take the emotion out of she's not getting the attention she craves from you for her 'bad' behavior. I would also make time when breastfeeding to read her a story - saying to her that she can choose a story IF she sits quietly and helps you turn the pages.. or that she can fold the laundry.. really, my DD loves to 'help' in this way - sure I have to do it all again.. but it's confidence building for her.. this gives her a bit of you and her time, but distracts her from the fact that DS being fed.
Don't forget to praise your lovely DD to the heavens if she goes to the loo and is 'well' behaved. Tell everyone what a lovely big girl she is and how brilliant she is at going to the loo, and get relatives to praise her on the phone when she's had a 'dry' day. (Ring them first to prime them to say that "she's done a great job at being a dry girl", and then ring them later to hear the 'news' of a dry day, and they can enthuse)
it is tough on you and I hope you're not too tired.. HTH