It sounds like she just wants to be little again. baby step brother probably gets a lot of attention for being cute. Maybe give her other ways to be a baby, like wrap her up in a big blanket and hold her and say"oh what a cute baby! you will be by baby forever, no matter how big you get you will aways be my baby" Allow her to regress a little, she will soon be back to her old self wanting to keep up with her big brothers.
as for the diapers, you could buy her her own box of pull us and say that she can wear them at home, but please pee in the potty so that you stay comfortable. you could also buy them a size or two too small so that they aren't comfortable.
she is only little once, don't push her too fast