V.W.
Merry Christmas!
Um, the first thing that comes to mind is that you have a 2 1/2 month old baby. I would guess that this latest Potty development is about attention. New baby, holidays, busy time... I did not go through this with my son (who was a little older when his sister was born- 3 weeks shy of his 3rd birthday), but I understand that regression is a common occurrence with the birth of a new sibling. If you are really having to stretch yourself thin to manage all 3 kids, maybe she feels a little ignored. If there is a way to give her some one-on-one, even 3 or 4 minutes a couple times a day might go a long way, if that is what is underlying this issue. Depending on her maturity level, you might even be able to discuss it with her. Not directly about the potty thing, but let her know that the baby's needs will not always be so high and soon you will be able to spend more time with her than right now in these first months of the baby's life. Let her know that you spent tons and tons of time with her when she was a baby because that's what she needed because babies can't do anything for themselves, and that right now her new little brother needs someone to do EVERYTHING for him... but he will grow and learn to do things for himself just like she has. Reassure her that this won't last forever. Soon she will be able to play with him... and you can start teaching her how SHE can do things to help her little brother too... like bringing a blanket when he needs it, or a bottle (if you use them), or bring washcloths when you bathe him, maybe singing a song for him when he is upset, etc... Little things that will help her understand that he can't do and she can do things that matter to someone else. She might become very nurturing, and forget all about the "potty" issues. Praise her when she is able to help you. And even though your older son might be more able to help, let her help when she is able so she can participate. Then praise her - not for what she does so much as for her willingness and desire to help take care of the innocent helpless little one. Praise her compassion.
As I said, my son was a little bit older when his sister came along, but he only was "moody" about it a VERY short time. As soon as I noticed his disappointment at being told "I can't right now b/c I'm ____ the baby" I had a short talk with him. Not short as in curt, but short as in not long winded. Very directly, I told him... " He got ALL of my attention for his whole life, but his sister will ALWAYS share with him her whole life. I love them both. She is helpless, so she needs someone to take care of her.. she can't even ___, but she will grow like he did and then I'll be able to ___ with him more often, and she will be able to play too! I know you are sad that I can't __ right now, but I still love you. But your sister needs to be fed (or whatever I was doing). Would you like to sit here with me while I ___? When I am done, we can ___." It never was an issue again. They are practically best friends now.. even with a 3 yr age difference.
Just be sure that if you promise to do something in __ minutes, that you follow through. The dishes/laundry/telephone solicitor can wait 10 minutes. And like with all the childhood dramas... this won't last forever.
Other than that... well, I hope some of the other moms have some ideas for you.
Happy New Year!