There are some books that are making the rounds among young parents I know because they offer solutions that work for all sorts of behavioral and emotional issues with kids, from toddlers on up to teens. My all-time favorite parenting book is How to Talk So Kids Will Listen, and Listen So Kids Will Talk, by Faber and Mazlish. Once you try some of the easy-to-implement strategies, this will become a resource you'll reach for again and again. The techniques are mutually respectful, affirming to the needs of both parents and children.
The same authors also have a companion book I haven't read that probably incorporates many of the same themes, but focus more on the strains that kids in multiple child families experience. That one is Siblings Without Rivalry. Since your daughter's potty behavior is very likely the fallout of her feelings about younger siblings arriving on the scene, I'll bet you'd do well to start there.
We use the How To Talk approach with my grandson with brilliant results, even when he's having occasional unhappy days or wants something he simply can't have. Once we find out what his issues are, as he perceives them, and let him know we truly empathize (even if the answer is still "no"), he's generally able to switch emotional gears pretty easily. He just needs a bit of extra attention and reassurance that we are really on his team.
I also agree that the PK teacher could become your ally, encouraging your daughter to use the potty at home.
And it's reasonable to show her how to clean up her messes herself. If you are patient, or better yet even cheerful, while you teach her, it will gently hand the natural consequences of the behavior to your daughter. She'll have a much stronger incentive to avoid accidents.
Give her as much positive attention as you can, T.. I think her main fear is being displaced, and if you punish her for misbehavior, she may think that's the only attention she'll be able to get from you. Just a thought.