My daughter is almost 4 and she was really good at going to the toilet. However, in the last month or so, she has started having accidents. This is only when she is at home with me.
My thoughts are your daughter is maybe a bit angry at you for having another new baby. Your best bet might be to ignore the accidents and let her take care of them herself, asking her to just tell you if she needs help. I believe many urinating and eating issues usually are issues of control. Tell her where the clean undies are and where to put the wet ones. Tell her you know she will stop having accidents when she is ready, and then turn it over to her. Hope this helps.