Potty training is such a chore!!! Good job getting her to the Doctor and getting the UTI treated. If she likes yogurt, let her have some - it'll help keep the diarrhea from being so bad.
The best thing we found that works with our 5 year old is the "potty first" rule. If she's coloring and decides she wants to watch a movie, "okay but potty first!" It doens't matter what activity she wants or if she wants a snack or drink - whatever it is, she can do it or have it as long as she goes "potty first!". That way, whatever it is she wants becomes the reward.
Then make a mental note of the time, and if in 15-20 minutes, if she hasn't gone, then "Hey, I need you to potty first before you do anything else!"
If it is a reaction to the new baby, then make sure you spend time with her, or find some way to give her positive attention too - she can help get the diaper or wipes for you, or can help prepare the bottle or get a burp rag - something, anything that you can include her and give her praise for helping out.
At night before she goes to bed, be sure to tell her 3 things she did that were good that day (even if she did wet her pants 5 times) - "Hey, that was a really big help when you got your brothers diaper! I really appreciated that. I really like it when you help. And you know how you went and got the pacifier for your brother, that was AWESOME! and you did a really good job eating your dinner too!"
Just some things that have helped us along the way....for what it's worth, and good luck!