A brother isn't an item just brought from a shop though ;). On a serious note though - no, it sounds very weird to say "newest brother" in English. You can say "newborn brother" if you just had one born into the family, but within some days it would change to younger (correct for this exercise's sentence) or youngest. The only context you might use "newest brother" (besides it being a bit of a joke about a newly born brother) in could be if you acquire one through adoption into the family and it's not the first time this happens. The first time you could conceivably say "he is my new brother", and for any consequitive brother you can then say "the newest".