I'm looking for both a rant & some advice from anyone who has been in a similar situation.
My sister & I have always been very close. Mainly due to the fact we've always been so similar in our outlook and values, raised by the fierce woman that was our mother. However in recent years, I feel like she has been replaced by an impostor & I don't know what to do.
Like me, she has always held strong, feminist opinions & has been completely dedicated to her career & independence. Then, she met her now husband. This was about 3 years ago. The whole relationship was a complete whirlwind - within a few months they were engaged and married the following year. Then, she fell pregnant shortly after and my DN was born a few months ago.
I admit from the start I didn't warm to the guy. Like my sister, he was in his late 20's and already pending a divorce from a previous marriage. This was a massive red flag to me, as was his explanation behind the divorce - his wife cheated on him, he was completely innocent in all of it 
My sister didn't seem too phased by it, she was so smitten by him, she dismissed all my concerns. I have to say, it was very unlike her as she was always very cautious in her previous relationships. She then proceeded to change even more. From someone who never used social media & had strong opinions on its use, she opened a number of accounts and joined him in posting every aspect of their relationship online. I mean, ever meal/trip out, every 'milestone' in their relationship was made public to the world, together with gushing messages dedicated to each other.
My DH & I, noticed from a start he was a heavy drinker - any family occasion ended up with him drinking himself to sleep. Unfortunately this was again dismissed by my sister UNTIL DN was born. When they stayed with us this Christmas, she lost her temper with my DH who suggested another drink. I asked her what her problem was and she snapped that she "doesn't want to deal with his [her DH] drunken aggression again". I was taken aback by how blasé she was about saying this. I obviously wanted to address this with her, to understand the extent of this behaviour but she stomped off to bed and wouldn't talk about it the next day, claiming it was nothing.
If that wasn't bad enough, she confided in me the other weekend that he admitted to a couple of one night stands he's had during their relationship. Again, I had to pick my jaw up off the floor. I looked at this stranger in front of me and did not know what to say. If she was my sister, she would have shown him the door by now. Yet here she sat, telling me how they're thinking of counselling to address these 'issues'. The guy drinks, cheats and generally does fuck all around DN & the house BUT she is adamant she wants to stay and fix it.
Am I going mad or has my sister had a personality transplant? What do I do? I know I'm supposed to be supportive but I honestly can't sit there and watch her be miserable and bring DN up around this waster.