Pregnant but this is about my too year old?

I have a 2 year old and she is being potty trained....The problum is she shares a room with her one year old sister and at nap time she takes her cloths off and wipes poop everywere and feeds it to her sister..i have tried puting her cloths on backwards she still takes them off i dont no what to do i asked my doctor why she is doing this and she said some kids do i need help i cheek on them every 15 mins please help