Children can be so sensitive. She maybe does not feel in place while at her moms place. Maybe the place is crowded. When she is in your place you treat her differently. She is alone. Maybe she likes it that way. Now tat the environment is different. With the many other children her mother has. She might be feeling out of place. If she acts well with you. Then her mother should be the one treating her bad. Or if not. Then she just prefers your company more. That is why she acts well with you. It might be a message she is sending to you guys. That she wants to spend more time with you than the mother. If she could be schooling from your place. She just wants attention that she is not getting from her mother. Otherwise I hope the psychiatrist can help.