Is it normal for a so called best friend to be that two-faced?

This all started not long after I got engaged to her cousin. We started planning our wedding, I didnt tell her everything we were doing for it, it was still early days of planning, she then announced her wedding and started buying stuff, I was happy for her, everything seemed fine until recently when I caught her talking crap about me and my relationship with my future husband on one of her stupid internet pages. She was accusing me of all sorts like copying her stuff which I would never do I'm strong and know my own mind I've always been like that so the last thing I'd do would be stealing other folks ideas, our dresses are by the same designer but I pciked it first and knew about the designer before her yet she's twisted it around on there making out like I copied that from her too.....its so ridiculous but it angered the hell outta me, I confronted her about it, she gave some kind of appology, at the time I thought it was genuine but she's said worse on there not long after that. However that was back in December and alot of things have happened since then, we do still speak, to my face she has been acting normal again but am I right to be so angry still? Part of me wants to cut ties part of me wants to forgive her for now and then if she does it again cut the ties. I don't know what to do, thing is we've been through so much together she was there for me when my Mum, Dad and Grandpa passed away, we've been friends for 13 years. There's alot of history there. She talks about her other friends behind their backs to usually moans about them infront of em and my partner and I would not be surprised if she's put daft rants on there up about her other half, she's alwasy nagging him too he just gets ignores it most of the time or talks back. She said to me this started because she was worried that we were having a simillar theme and she didnt want folk thinking she copied me if I got married before her (our dates changed now though so I'm getting married after her), I told her at the time we're both having black feature in our wedding but we've done it in different ways, no-ones copying anyone so what's the problem, she wouldnt listen though I honestly believe she would have wanted me to change my entire theme to some different colour all together to make her happy. She is the type to huff and puff if she doesn't get her own way. I told her to not be so daft worrying about silly things but then it's just escalated to her then going online and saying sh*te behind my back. Don't want to repeat myself but there were some utter nasty things on there that were ridiculous but ofcourse the folk on there are listening to her without knowing me or what I am like. I was tempted to put my side up there but that would make me no better than her. I've spoke to my partner and my half sisters about it, they say to see how things go for now and if she does it again cut ties. She's been acting like my best friend again since. How do we get past this?