I am so so so so sorry I haven't been active lately -_-
But hey!! This situation is totally salvageable!!
I would not tell Liv- you have multiple friends, of which you need to keep their secrets. I would really have a consversation with Liv saying that you would love to tell her, but you can't because you would be violating Ally/ Arie's trust. To be brutally honest, it's not Liv's problem (AS CRUEL AS THAT SOUNDS :(((), but it's true. You can MAYBE say it's about a boy, but other than that, I wouldn't tell her.
You keep Liv's secrets from Arie and Ally, right?? So why can't they have theirs??
Listen, this is a bit of a pickle, but I think you all will make it out unscathed ;)
Best of luck girl, and I hoped this helped a bit!! :)