"They are equally responsible."
Translation:Ellos son igualmente responsables.
I was wondering the same thing. To me, if the sentence is saying the people are equally to blame for a problem that happened, the problem is probably a temporary thing that will be resolved. That makes me want to use estar instead of ser, but I am not a native speaker.
I could also see the phrase being used for longer term things, such as saying that a mother and father are equally responsible for their children, or to talk about responsibility for a problem that will have long-lasting ramifications.