The use of "的“
First of all, it's great news that chinese is out! It's a very fascinating language. I've been learning it for about a year now. I took the chinese test and I got a couple of things wrong- the use of "的". I was taught (by chinese speakers) that we don't need to use it if the relationship between people ( eg. mother, father, wife, brother...) is close so 我妈妈， 我爸爸， 我哥哥， 我妻子 should be correct. To my surprise duolingo didn't accept this answer. So what's up with the "的"?
It's a mistake. Lesson notes (https://www.duolingo.com/skill/zs/Family-1) also mention that 的 can be skipped:
"For people that you have a close relationship with (e.g. family), and for institutions and organization you are involved with (e.g. work, school), you can choose to leave out the 的, but this only happens with pronouns."
Because the 的 is used usually in these situations, Duo can't tell. However, these days most Chinese speakers drop it, like me. You usually would, in fact, say 我妈妈 or 我家 and things like that. Duo isn't aware of this, and you can't blame the team for the course is still in Beta. Report it!