"They like to sing with a brother."

Why isn't "Їм подобається співати з братом?" accepted

While the English sentence is technically correct grammatically, I can't really think of any occasion when I'd say that. It's the equivalent of this Ukrainian sentence: Вони люблять співати з якимсь братом.
I believe that in this instance you need a possessive pronoun- ie my brother, his brother, their brother. A/an are indefinite and simply don't work here. This also goes for the other sentences here on Duo where we have to write a father, a mother, etc.... It's nitpicky. But real.