Translation:My sister got up today at 8 o'clock.
I am surprised that the suggested correct answer substitutes the numeral 8 for the word, although "eight o'clock" was accepted too. Rightly or wrongly, I have always considered it good practice to retain the convention used in the original when translating, so that numbers spelt out in words usually stay that way. Does Russian ever cite the time in numerals?
I already knew this from previous lessons, but I rather like it for using Duolingo - saves me some time at least. However, I wasn't really paying attention to the written text, and just translated what I thought I heard. It actually counts '7' correct as well, which it really shouldn't.
The word order sounds peculiar. "Today my sister got up at 8 o'clock", or: "My sister got up at 8 o'clock today", or even: "My sister got up today at 8 o'clock".
So the word order is quite flexible, but: "My sister today got up..." is the one version that sounds rather unnatural.
I think it's that you have separated the verb from the subject. Note that in all the examples I gave, "sister" stays with "got up".
I'm don't think there's an absolute rule against it, but it sounds odd.