My son doesa the same thing, and he has been potty trained for several months. For us there are a few issues behind this: jealousy of the younger sibling getting attention, unwillingness to stop playing before its too late, and plain old toddler defiance. A general rule for toddlers seems to be if you want them to do it, they will do everything else and laugh about it.
I would say try to keep a small notebook with her accidents listed, along with her mood, her sibling's mood, and anything that was going on at the time. Maybe when she gets over tired this happens, or maybe she wants to do something but can't. There could be several easons behind this, even as simple as she wants to see what reaction she will get out of you. May times kids act out as a sort of test, to see if you will still love them no matter how they act.
Of course, you also have to take intoa ccount that she has only been trained a couple weeks now. Just like any other new thing (feeding herself, dressing herself, etc...0 there will be backslides as she works towards really mastering her knew skill.