This sounds psychological to me, esp. if she was completely potty traind for a year previously. It looks like her little sister was born when she was 3, is it possible that that could have contributed to the regression? If she saw the baby getting attention everytime it needed a change she might have thought "if I need a change too, I will get that attention". If so, at this point it's probably just a bad habit.
I would approach it from a completely accepting position, and just ask her why she is not relieving herself in the restroom- be honest, tell her that you're concerned for her health and well-being. If she says that she wants to and can't control it; see a specialist to find possible physical causes. But if she's "too busy" dig deeper; maybe she is embarassed to admit a physical condition or maybe she is trying to cope with some trauma- keep in mind that trauma is subjective so it may be a very small thing that has been bothering her for a long time. No matter what don't persecute her- it's the behavior that she needs to change, and she needs your love and support to do that.
Guided meditation may help you help her understand the root of this issue and resolve it- I agree with Crista that this should be resolved soon before it becomes too deeply imbeded into her sub-conscious.
I wish you the best.