I'm having the same issue with my boy(tho he is a bit younger, 2yrs), he was going so well dry all day and even some day sleeps, now he's decided he won't go to the toilet at home. He will go when were at swimming lessons and at Kindy.
I've decided to take a complete break from it to take te pressure off and have him outside as much as possible as he is quite happy to 'shoot rocks' LOL I know thats probably a boy thing but at least theres no mess when there outside.
It definetly could be she is starting to stress about having to share you so maybe lots of special mummy time without toileting pressure might be an idea. Keep reassueing her as much as possible that the new baby is never goin to take her place, get her to help out as much as possible when it comes to getting ready for the baby. eg decorating babies room.
Instead of having a tug of wills about the toilet maybe have potties around the house as a reminder to her, I know this may feel like a step backwards but without pressure she may just start using them herself.
At least it's summer so lots of outside play will save the floor LOL
Good luck with it and if you find a magic wand that fix's it please let me know smile
Take Care Gem, Mannix and Colton