Yes, comma is widely used in Swahili, same way as in English. What's happening here in the Duolingo course is just bad punctuation. I've seen several sentences started without capital letter here, so it clearly needs a read-through despite they've taken a long time to release this course.
Before you start worrying about starting a sentence with a capital letter, or putting a comma or a question mark, you need to know what Duo does when he checks your answer. He removes all your punctuation, and changes all your letters to lowercase, to see if your answer matches one of his set of possible right answers. You could even have written:
hOw iS, hOmE? BROTHER!
which matches his
how is home brother