Long post, apologies in advance.
We are not close. We never have been. She was very close to her own mother and was devastated when she died a couple of years ago.
Since then, she's trying to lean on me more and more, act like we're bosom buddies, but the issue is she's never put in the effort. I am 30 years old and I just want her to leave me alone. I went through a phase of trying to forge a bond with her, going places, calling her to chat etc but it would always backfire because the topic always came round to my dad.
My dad ruined her life. He was abusive. She was jealous and possessive. I didn't have much peace at home growing up and I was out asap. Then, 3-5 years ago he told her the marriage was over and she's refusing to accept it. She wants me to help, but I don't know what she wants from me. I've advised her to seek legal advice because this isn't something that I can help her with. She doesn't want to be "in the system" with a law firm 
We often get into awful arguments because of her unreasonable demands or expectations; she doesn't seem to see or accept that she's in the wrong, and nothing is ever her fault.
This weekend she was here to drop off flowers and presents for her sister who is staying here with her DC for the time being due to other unrelated life catastrophes and I debated telling her then, but she opened my cousin's door to have a nosy and argued that it was fine because she was just having a look and she didn't step in to the room 😔 we had an argument and she started crying and was bringing up examples in the past of me being rude and mean to her and how I'm always smiley and happy with my dad.
She then went home and sent me a spiteful and manipulative text message.
I am tired and I don't want to share the news with her, or any of my family, quite frankly.
I suspect aunty and cousin will find out or guess first as I've been sick a few times - I've also had a cold and blamed it on that, but that's not going to fly forever.
Anyway, I'm thinking of putting this off until the 12 week scan and making an announcement then, but I can predict with a high degree of certainty that she will be upset and kick off and try to kick her sister out of my house and I just want to enjoy this pregnancy instead of worrying about stupid arguments about shit she's concocted in her head.
Sorry for the long winded post and thank you for reading.