Ladies any advice for me?

Ok I've know this girl about 15yrs since school days... but she is sly it's not the kind of person I want to be around now but she's married to my brother and is it wasn't for that I would have stop talking to her ages ago.... it's the type of friend that don't got your back she would talk you to other people and I hear it back I've spoken to her about it but she does it to everyone... I've done alot for this girl and she dogs me like that... she's got three kids two is blood niece and nephew her other child we treat as our own... how do I cut her off without it affecting my brother or see the kids???

Sorry forgot to mention she was seriously ill and I had to help her out and she still does that to me