Please do read up on encopresis.
yes she does, she does it anywhere she goes x
If she goes to her Dad every weekend does she have the same problem with him?
she is great with my daughter, and i think she really clever so i dont think shes doing it because of her younger sister. she gets one on one time with her dad because she goes to his every weekend, and has done since she was 4 months old x
http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0002537/
is a good description of this kind of problem.
It could be she is upset by something , how is she doing at school,has she shown any jealousy about the little one , perhaps she sees baby pooping in the diaper and is doing it to get more attention .Make sure she gets some one to one time with you and her dad ..