Sister in law advice...😱

So, I'm due Sept. 7 with my first baby, super excited!! A little worried about how I'm going to handle my husbands family visits. Ever since I met him his sister has been a total b**** super possessive of him like he belonged to her. She has serious problems relating to people as she and my husband are adopted from different parents, but she has a bit of a trauma with the whole adoption thing even though she was adopted when she was 2 days old. Anyways, I've been married for 2 years now, dated him for 1 and she has always been mean and distant and I've never been rude to her or anything. 
Now that I'm pregnant and expecting a baby girl she pretends like she's my best friend and has said comments like "ill sleep over to be with her". 
Imjust really annoyed, I really dislike this girl and don't want her around... She's been very very mean, not once apologized for her past behaviors... (Made my mother in law hate me for over a year cause of a lie she said) and now that I'm having a baby she pretends to be the nicest person ever and expects me to forget that she was a total b**** cause she being a over cookies to my house. 
I'm so over his family, his mom is super annoying always meddling. And now she is starting to hover over like the baby belonged to her. 
I don't know how to handle this, I've been controlling my anger for all of this time but now with my baby coming I'm afraid that I'll explode!!! 
What to do... I need her to understand that in life you get what you give and that we are not close!!!!