I apologise in advance for the length.
Early last year my younger sister announced that she was pregnant, everyone was shocked as she'd always said that she was never having kids. It turned out that the pregnancy was accidental but nonetheless after the initial shock I soon got over it and I was happy and excited for her and her boyfriend.
A few weeks later we (me and my husband) visited my parents and my sister etc for the weekend. I'd moved away from my home town to another county to live with my husband as he lived in a more populated town where everything was easily accessible and there were more job opportunities compared to my home town etc. However we visited my family regularly and kept in touch via Facetime and texts etc. We took everyone out to lunch, my husband went out with my sisters boyfriend and it seemed okay - no weird atmosphere.
As the pregnancy progressed everyone kept in contact with my sister including my husband however I noticed that she was becoming rather short with me on occasion but I didn't think anything of it. I have a lifelong health problem that needs to be managed every day otherwise it will lead to complications and I'd become seriously ill but she had never asked how I was or how my husband (he's disabled) was. At one point her boyfriend moaned to our dad that I hadn't asked after her one day and my dad turned round and said X and X have lifelong health issues, have you ever asked how they are?
Anyway she carried on being weird, the day of her 3D scan I was at an event with some members of my husband's family and I told her that I'd ring her after as didn't want to be rude to the hosts and sit on my phone. She texted me a few times that day accusing me of not being interested, again I said that I'd call her later and when I did she ignored me. I text her politely saying WTF and she never responded so up until the birth I only made sporadic contact because she wasn't responding and I didn't want to stress her out by having a row or something.
After my niece was born we both text her and her boyfriend saying congratulations and that we were glad everything went okay, to be fair to her she responded a few days later really nicely and I thought maybe we could have a chat and maybe sort things out soon. I text her a few weeks later asking if I could call her to do exactly that and she responded saying that I was a miserable bitch and clearly unhappy with my life and that I was never going to meet her child, by then I was 
because it came out of nowhere and it wasn't true so I didn't bother responding as it would have only made the situation worse.
My husband attempted to find out what the problem was via the boyfriend and the boyfriend text him back one Friday night and was very aggressive and rude, again my husband knew better than to retaliate so he politely text him back and blocked him.
A few months later, aware that my dad was stressed by the situation between me and my sister we arranged to go down one weekend to visit family members and to hopefully talk to my sister, I arranged to see just her - not her boyfriend and mum was going to babysit my niece# round my nan's house, my dad had to ring my mum to make sure that my sister wasn't in their house before we could go in and unpack 
So I went to see my sister, she sat there in her living room looking smug with herself and before I could say anything she viciously attacked me by saying one thing after another - that I'd married a baby as my husband has a mild disability (he's actually independent and works full time etc), that I'm jealous of her lifestyle and her boyfriend and that I'd become arrogant and rude. I stood there in stunned silence and then she asked if I wanted to be in my nieces life?! I called her a manipulative bitch and a cunt before storming out.
I went back to my parent's house and told my dad and my husband what had been said, my husband was understandably angry and my dad sat there in silence, I've now blocked my sister and her boyfriend on everything and I never want to see them again.
I'm unsure how I'll get past this as my sister has always been vindictive but never to me until she became pregnant and then it seems like all hell broke lose, a few family members have confided in me that they think she is mentally ill but I feel like they are making excuses for her behaviour and if she is ill, why is nobody saying anything to her? Even my parents are reluctant in case she stops them seeing the baby so of course they are pretending that everything is fine.