J.P.
I think that you should've never said anything to the other mother, and watched the situation closely yourself. Most likely the "step-sister" is probably feeling left out and that is why she is acting the way she is - and you are just further making her feel left out. You should also teach your daughter that she needs to be considerate of other people, and there are reasons why people (especially younger siblings) act out. It is also normal for siblings to argue, and fight, and sometimes be mean to each other. I am sure that there are also times (most likely when your daughter is not around) that the sisters get along well. Maybe when your daughter comes over and her older sister is now ignoring her, she is acting out to get the attention. She probably feels like a 3rd wheel. I would've encouraged my daughter to try and include her in activities instead of pushing her out!