"My family is really happy."
Translation:Gia đình tôi thật hạnh phúc.
I think the English translation here confuses many. This time, when they say "My family is really happy." They are using "really" in its form synonymous with "truly". So, the more clear way to phrase what they are actually saying here in English is "My family is truly happy." That is why the Vietnamese translation is "Gia đình tôi thật là hạnh phúc" instead of "Gia đình tôi rất là hạnh phúc." Thật means true, thật là means truly (really).
THAT answer listed above is NOT the answer they gave in the lesson. I would accept THAT answer much more readily, rather than the one using là after the word thật. If we are going to learn this language, it's time Duolingo stopped using idioms and started being consistent with actual Vietnamese phrasing and word order.