it is yet unknown whether the event in question will happen or not ( usu. the implication is that it will not happen)
that (it) remains to be seen
it's any body's guess
that's () an open question
it could go either way
we'll (just) have to wait and see.
«Мы после обеда засядем в ералаш, и я его обыграю». -«Хе-хе-хе, посмотрим! Бабушка надвое сказала» (Тургенев 2). "We'll have a round of whist after dinner, and I'll clean him out." "He! he! he! We shall see! That remains to be seen" (2b).
«Было время, товарищ Воробушкин... тобой детей пугали. Один бы ты и в жизнь не отплевался. Да и была бы жизнь, тоже бабушка надвое гадала...» (Максимов 3). There was a time, Comrade Vorobushkin, when your name was used to scare children. On your own, you'd never have wriggled out of it as long as you lived. And whether you'd have lived at all is anybody's guess..." (3a).
...Тех — миллионы, которые не позволят вам попирать ногами свои священнейшие верования, которые раздавят вас!» - «Коли раздавят, туда и дорога, - промолвил Базаров. - Только бабушка ещё надвое сказала. Нас не так мало, как вы полагаете» (Тургенев 2).".. .The others are millions, who won't let you trample their sacred traditions under foot, who will crush you!" "If we're crushed, serves us right," observed Bazarov. "But that's an open question. We are not so few as you suppose" (2b).
"...Ваша власть и держится-то тридцать годов ( substand = лет), и сколько ещё продержится - это ещё бабушка надвое сказала, а наша вера две тыщи (phonetic spelling = тысячи) лет стоит...» (Максимов 1). "Your power's been in existence thirty years, and how long it'll survive we'll have to wait and see, but our faith has lasted for two thousand years..." (1a).
Большой русско-английский фразеологический словарь. — М.: ACT-ПРЕСС КНИГА. С.И. Лубенская. 2004.