You didn't mention if it happens when she's busy playing or watching TV or using the iPad. If it's any of those instances it could be that she doesn't want to stop doing what she's doing. You could tell her she'll have to clean herself up when this happens, then take her from whatever she was doing, have her undress, rinse her own panties and pants in the shower, then take a short shower to wash herself up and dress herself in clean clothes. Once she realizes she's missing out on a fun activity she may try harder to get to the bathroom. And go back to the basics, taking her hourly to have her try, particularly around the time she has her mini accidents.
Kids are funny, too, in the sense she may be absorbing the fact that there will be someone younger than her coming into your lives and this may be her way of dealing with it. Be prepared for major regression once the twins arrive and know that it is very common, it can happen in children even older than she'll be.