Translation:I was working with her brother then.
“I used to work with her brother.” Should be an acceptable translation. The “then” is redundant. In English, “used to” would be the most typical translation for “tehdy”. This course does not accept it in any module without “used to then”. Vladu: not every sentence using tehdy in the course is referring to a specific period of time mentioned earlier.
"Tehdy" refers to some past time which is understood from the context. Explicitly mentioned or not. Tehdy here is a direct counterpart of the "then" in the English sentence. If you wanted to say “I used to work with her brother.” you would say just "Pracovala jsem s jejím bratrem.".
You can link to some other sentence where "tehdy" is used as "used to" instead of "tehdy", and we will see, but I claim it is not as direct translation as the official one. And therefore should not be accepted here.