SUBREDDIT: r/relationships
TITLE: Me [32 M] with my fiance [27 F] my mom keeps contact with my ex wife's child and my fiance is furious about it!
POST: I have a great relationship with a girl who I want to marry. My ex wife cheated on me and tried to hide it. We're divorced now.  I had kept in touch with my ex wife as friends and sometimes talked to her child about games etc.  Since I got a new girlfriend she did not mind me sometimes communicating with ex wife, but the child she felt threatened by.  So I gradually stopped contact because what reason to communicate with ex anyway, as now we're more serious and want to start a family with my new girl. 

But, the only thing that we fought about now is that we found out that my mom still communicates regularly with my ex wife's daughter.  My fiance is threatened because she thinks they talk about our personal life (we don't know but it's her thought,  maybe insecure but also a but understandable in a way).  Women can be very competitive especially my fiance. 

A few months ago my ma said she stopped.  But my fiance found something on social media and now she's furious.  My ex also sent a fake message from a fake profile to me making it seem  like I'm flirting ng online.  My fiance now wants nothing to do with ex wife and her child, and told me that my mom must stop or get out of our lives forever. Meaning no contact ever. 

Is this extreme or justifiable?  My mom has a history of gas lighting and lying, sort of a borderline narcissist (but she's very friendly, just lies a lot).
TL;DR: 
just like the title says,  my mom keeps in touch via social media with my ex wife's daughter. Should we cut her off?