Certainly "He is my newest brother" means the same thing, and in my opinion even translates more directly.
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".
The expression "irmão mais novo" does mean "younger brother".
If you want to say the weird "newest brother" (as if you're getting a lot of brothers and this is the your latest "achievement"), then you'd say "o meu mais novo irmão".
I could see the more literal translation (newest) being used in more modern family structures (adoption, remarriages). Seems like it should be accepted, with a note that the alternative translation is "youngest".
The expression "meu irmão mais jovem" is weird. It's not wrong, but it's weird.
I really don't know...but we also use "velho": "ele é meu irmão mais velho/novo". And we use it normally...
Looking back at this now, it would seem that because "mais" means more and not "most" that "younger" would work better than "youngest"
Both are correct.
- Mais novo = younger
- O mais novo = youngest
Now, since you have "o irmão" (defined by "meu"), the distinction gets a little blurry. But "younger" is indeed the best option.