This is P., I have been raising kids for 27yrs, First off she has you pegged, she knows this upsets you and is using it as a attention getter from you.. you focus more on her and so dose everyone in the house...
How to solve the problem.
1. stop giving her underwear.. when she gets home take off her underwear, take it out of the draws. It is a privilege to use the toilet, not not a struggle. When she starts to realize that you will not put up with it any longer she will start using it.. you have tough her to dis behave and so this is her rebellion over you.. which later in life she will also use other things as Eating distorter, and wetting the bed. when she dose use the toilet give her only one underwear. let her work to get it back.
I am 45yrs old I am helping to raise my grandchild 2 yrs old she has been potty trained as the old days at 18months old, even i have had this problem with my daughter child, and this works with every child. even yours... I just did this in the last 6 months. Now we are working on night time training and so far.. only 2 mishaps. not bad..... my methods are harsh but we women have been doing it this way since 1900 that I personally know of. My great great grandmother had my father potty trained at 16months old, and i was potty trained at 19months old my daughter was 18months old. so My ways work.
When a mother gives her child power over the family... The child loses control.
Do not pay attention to her during this time, do no look her in the eyes, do not even talk about it.. just clean up the mess and walk away go into another room when you are done. go read a book act like nothing ever happed..
as for your older daughter.. make sure she gets to do something like make cookies with daddy.. NOT YOU... DADDY.. you are the reason she is doing this.. So let a father, take his place in the house hold. let the older daughter and her father go out for Ice cream..
and you sit in a chair read a book, let the little girl just do what she normaly dose.. it will dawn on her and she will start using the toilet.. she will want the ice cream with dad an sissy, or you to spend time with her..
Just remember.. there is no talk ever about this again in the house hold.. let her be...... don't even let her hear you talk on the phone, or in person with friend, or relitives.. the word is mumms.. This will help..