If they did have a bug, she left them with someone yesterday so she could head out, and then had another child in the house today. So, she exposed whoever minded the twins yesterday, the crowd she was out with, the other child today, and potentially whoever that child is in contact with, to the bug. Selfish on all counts.
If there's even an ounce of truth in the story she told about the spellings, she should be reported to whoever employs her, and that child's parents also. Even if the only ounce of truth is that she helps kids with spelling at any point, as hers is atrocious!