Could an extended house guest be causing my 2 year old to act up?

My sister Martha moved in with my husband and I about 6 months ago. My 2 year old daughter has been very disrespectful to her from day one, which I try to stop buy Martha would say shes just playing. She has become very willful and mean to every one. Martha means well but tries to be the mom and it only provokes my daughter to fits. Martha says the problem is that she isn't disciplined consistently. Even though Martha is loving and good to my daughter, she seems to constantly irritate her at the same time. Is there any research on an extra person living within a family TOO LONG causing a young child to be confused or threatened or anything? Maybe it isn't so much our parenting causing the attitude in the little one but rather a sister that needs to know when "a few months stay" ends and start her own family.