It's a more colloquial form. Like in English we can sometimes drop the -ly in adverbs even though it may not be technically correct.
Why use a colloquial form when in the course, they used сильнее? It's like me teaching slang to a Russian who wants to learn English.
These are pretty much interchangeable, it's not nearly as bad as mixing adjectives and adverbs by people who speak their native English "real good".
I wouldn't call them synonyms - just two acceptable variants of the same word.
duoling -- if either word is OK, then please accept either сильнее or сильней. Also, can you delete responses which are totally off the subject of the sentence? Political discussions should not be allowed here.
Just to elaborate: "свой" is a reflexive pronoun that refers back to the subject and hence cannot be a part of the subject (свой брат) itself.
Сильнее is another acceptable form -- read the discussion at the top of this thread.
I received the English sentence here to be translated into Russian. I was marked wrong because I said сильнее instead of сильней,, which I had never seen in this context. After reading the discussion here, I tried to report my answer, but the option "My answer should be accepted" was not offered.