<human>: Can you give me a shortened version of the text?

Me: F21
Sister: F27 
Mum: F59

It's hard to keep this short but I'll try, my sister and mum had a massive falling out around 4 years ago, entirely my mum's fault, they've entirely cut contact. I'm still in contact with my mum although I live far from home, we clashed badly in the past but since I moved out our relationship has been a lot stronger. My sister and mum still live in the same town. My sister continues to want nothing to do with my mum, however whenever I visit home my mum constantly hounds me for updates on my sister, I keep these very vague as per my sisters wishes. 

Here comes the problem, my sister is 8 months pregnant. My mum didn't know, and I was told by my sister NOT to tell her. I haven't told my mum anything, but she's managed to find out through someone else somehow. My mum has messaged me via Facebook and is trying to make me feel bad for not telling her.

 I've not responded and have no idea what to say. Today is my sisters birthday so I don't want to tell her that mum knows until tomorrow, if at all. I don't know how to respond to my mum. She has a dedency to over react very badly (has kicked me out in the past for getting average grades, for coming home slightly tipsy, etc etc)
<bot>: My mum and sister cut contact years ago, mum found out sister is pregnant and is extremely mad at me for not telling her, even though my sister explicitly told me not to. Don't know how to respond.