Translation:My relationship to my brother is not so good.
I reported this because relationship with sounds much better in English. Relation to sounds more like a familial relation only (you wouldn't say relation to my friend, for example). They are both accepted, but to say one's relation to someone is not good sounds very odd to me.
Isn't it important to learn about the pitfalls too? I appreciated it a lot in the earlier lessons when I was forced to remember, say, which phrases use a definitive form in English but not in Swedish and so forth. It's aggravating but useful. Now that we have a slightly incorrect official translation it doesn't underline the difference and doesn't alert the learner that there is something to be wary of unless their grasp of English is firm enough to bring them here. I would rather add this explanation to the actual lesson section (which doesn't exist for this category yet) and use the correct translation all the way through.
I agree here - it's important to point out the differences between languages, including with prepositions. For example in German, you say 'I interest myself for music' if directly translating 'I'm interested in music'. Surely it makes sense to highlight the areas where prepositions are different to those in English?
I don't feel that by using "to" you have made anything "easier" for learners. On the contrary, you simply irritate the learner, and lead him to believe that you don't know English. The English sentence is just plain wrong, and if you wrote the English sentence correctly, using "with," learners would learn that the nuance they want to express in Swedish would be done using "till." People with brains in their heads would notice what is going on.
Use "till" in the Swedish sentence, and use "with" in the English version. No one here is demanding that "med" be used in the Swedish version of the sentence.
I would agree that "relationship with" sounds much better. In fact, as alluded to by CrazyChao, saying "relationship to" doesn't really have a meaning that jives with the health of the relationship. My relationship to my brother is that I am his brother. This doesn't become good or bad in itself. My relationship with my brother can change depending on whether e.g. I listen to him or stop talking to him. So, I think that "relationship with" is really the only correct answer for this sentence and should at least be the default answer.
I do appreciate the explanation by Arnauti on the Swedish difference and I will try not to say that "jag har ett förhållande med min bror" :)