Translation:You are going to see your sister in a while.
I think the meaning is a bit different: "πρόκειται/going to" is more about a set plan or intention while "about to" is more about being near a time point.
In the introduction to the lesson "about to" is given as an alternative to the "to be going to" form.
Both of them are accepted here, but we're going to leave out "πρόκειται να" as a translation for "to be going to" in the new tree and simply use "θα..." instead, which is what's most often heard.
It should be accepted now in this sentence. Thanks!
It's not easy to find all sentences with a given word so if you use it in another sentence and it's not accepted, please send a report :) Thanks!
My "...in a little bit" was marked wrong, but seems like a good translation (bit is not in Greek, but the translation sounds better than "in a little."