Being ally with Co-sis?

Yesterday my co sis came home.She is my husbands cousin brothers wife. She is new in family and at the time of her marriage there were lot of fights since husbands cousin brother went against his parents wishes to marry her.Every one was kinda against her since they felt she stole their son and he could get much better girl.

When she came home she was very friendly with me.She was eger to tell me things but we coudent get alone time.So she took my no.

Now I know what it is.People are probably talking behind my back.Esp my SIL is very sweet to me on face but its artificial sugary talk which repells me.I know behind my back she is spreading venom.At a.time her 3 year daughter spills it innocently.My cousin co sis wants ally with me so she could discuss her.problems about in laws also she want to know what people are saying behind her back and in return want to tell me what's happening behind my back.

What shall I do.Shall I go ahead and listen to her ?or that will create more problems than solving any?

Being ally with Co-sis?