It is understandable, and as long as you use the right intonation it would even work in speech, but it is not very elegant if not wrong. The normal way to form a question like this in English is to use a version of the auxiliary verb "do" as in Duolingo's model answer. I guess Duolingo wants you to follow the norm. I believe "declarative questions" like the one you suggested are treated as second class citizens in the grammatical world.
if someone was telling a story and said he followed his brother, you could ask this question in disbelief. you followed your brother.
The individual word "seguiu" is pronounced correctly, but when listening to the entire phase the female voice pronounces the "g" like "gw" or something.
Concordei. Quando encontrei esta frase hoje (2018-11-28), ouvi a voz feminina dizendo «següiu». Esta pronunciamento da palavra é usada numa região do Brasil? Repito esta pronunciamento, mas estou curioso se esta é comum num lugar no mundo lusitano. P.S. Esta mulher tem uma problema comum com a letra «u», há muitas vezes quando encontrava uma frase com a palavra «quem» e ela disse «qüem».
In addition to what Davu said, it could mean, "Did you follow her brother," or, "Did you follow their brother?"
Yes, although without context "your" seems most natural. To make that interpretation the only choice they could have used "Você seguiu o irmão dele?".
then why don´t correct the answers and include all possibilities? Or, the ideal is change the question and formule "Você seguiu o irmão dela?"
That's a question for Duolingo.
This problem of ambiguity comes up all the time and it seems to me that once you know about it you can make life easier for yourself by using favoured patterns. For example, here "your" seems best, but in a question like "Ele seguiu o seu irmão?", "his" is probably the best bet.
If you know your answer is correct, though, whatever it is, ask for it to be accepted and eventually all the options will be covered.
i don't understand why the portugese sentence is "Você seguiu o seu irmão?" and not "Você seguiu seu irmão?".
Could somebody explain it to me please ?
Both are correct.
You asked a similar question here: https://www.duolingo.com/comment/13648079 and the explanation is the same, but perhaps Ruama's comments in this discussion will also help with some other situations: https://www.duolingo.com/comment/8843937
By the way, Ruama's opinion is that it's (almost) always better to include the definite article before any possessive adjective: https://www.duolingo.com/comment/5412105