Hello and Good Morning. Your daughter may be the daughter that your sister wished she had as her own. She may be trying to put ideas in your daughter's head so that your daughter may one day say, "I want to live with my aunt." Since she already spends a great deal of time over her house, then your sister may hope that you will comply.
If you thought that you and your sister had fairly good relations, then there is no reason why she should be putting your daughter in the middle of your relationship. It will be difficult to change your sister. But what you can do is to disallow your daughter from going over to your sister's house for a while and then see how your sister responds. Then you can have another talk with your sister. She does owe you an explanation for her unkind words about you to your daughter.