I need some help? (Preferably from a parent or an older sibling) What would you do in a situation like this?!?

[The Mystic]

Okay so my little sister is 12 and i'm 17.
My sister made a twitter account and told me not to tell mom about it, so i agreed with her. Although she doesn't know that i check up on it everyday. I keep my sisters secrets because i want her to know she can confide in me, however i saw something on her twitter account earlier that i did not like and i think i should tell my mom, however i don't want my sister to think she can't trust me.
She tweeted, on a public account which she has more than 3 thousand followers on that my mom's boyfriend just got out of jail and he's abusing my little sister again. My mom's boyfriend is such a lovely man and has never touched a hair on my sisters head (they haven't even been in a room alone together!) And he's never been to jail either! She's tweeting this on a public twitter account!
I want to tell my mom, but i don't want to break my sisters trust. But my mom has always told me that between me and my little sister, i'm the responsible adult which means i have to make the right decisions.
I need some help here, how should i go about it? I don't wanna just ignore it because making up false things like that online can get someone into a lot of trouble!