I understand the point, but I don't think this is entirely correct in English, where we often use a negative singular and plural interchangeably. For example, if I don't own a cat, I can say "I do not have a cat" or "I do not have cats" to mean functionally the same thing (that is, zero cats). There might be a few cases where you would want to distinguish between not having one cat and not having multiple cats, but for the most part you don't. So, if the import of the German sentence is zero cats, then it seems to me that we should accept either a negative singular or negative plural English translation.
Exactly. I can't count how many Duolingo phrases I've had to translate into their logical equivalent, because the literal equivalent isn't available as a choice. This question, however, requires the literal translation. This kind of inconsistency is marring the quality of this (acknowledged free) service.
The only situation I can think of in English where I would say, "She doesn't have a brother" rather than "have brothers" is if I was correcting someone.
"Doesn't her brother live in Alaska?" "She doesn't have a brother."
Which works because there was already a reference to a singular brother.
I'm not understanding the keinen on singular Bruder vs. keine on plural Brüder?
Bruder is masculine, so you need masculine accusative keinen in Sie hat keinen Bruder. = She does not have a brother.
Brüder is plural, so you need plural accusative keine in Sie hat keine Brüder. = She does not have any brothers.
I'm not sure which part of this is difficult to understand? Can you be more specific in your question?
As for whether to use "a brother" or "any brothers": both are grammatically correct and it depends basically on how many brothers you would expect her to have.
In countries where people usually only have one or two children, singular might be appropriate as most people will only have one brother if they have any brothers at all.
In countries where people often have four or more children, plural might be appropriate as many people will have two or more brothers.
the "ru" is pronounced like "rou" in "router". Just don't forget that it is followed by a "d" in "Bruder" and not by a "t" like in "router".
EDIT: For all the people out there who don't pronounce the "ru" of "router" like the "ru" of "ruder" (rude - ruder - rudest), please pronounce it like "ruder" like mizinamo suggested below.
In the last session it was : Sie hat keinen Brüder - She does not have a brother In this session we have : Sie hat keinen Bruder - She does not have a brother
Is there some explanation. Perhaps in English we should have bröther meaning both brother and brothers just to confuse everyone.