When we speak about people we do not use "的".
It depends, if it involves close relationship or ownership you don't need it, but by now we can just all simply expect duolingo to expect to have the 的 with everyone
You can also say 我妈妈， in fact, it sounds even better
When referring to family members, the 'de' is frequently dropped.
If you're talking about a person in your family or a close friend, someone you have a close relationship with, you don't really refer to them using "的“.
de is more for when you need to refer to someone that is outside of your family or who you are not close to, if you are reffering to your mother you use the phrase "我妈妈“ de can be omitted
Yixie = some
Or colloquially, a couple, a couple of, or a few.