"I love my younger brother and sister."
maybe it's because i learned chinese from 3 other apps before duolingo released this course but "我爱我的弟弟和妹妹" feels..better.
I've heard that when talking about your family members you don't need to say 我的 it's redundant, although I'm not sure
yes, adding 的 after everything is redundant and it is more often dropped from speech in colloquial Mandarin.
弟弟 and 妹妹 already refers to members of your family and already implies the "my". So your sentence could be understood as "I love my my younger brother and my younger sister."
I think that it's like saying ‘I love Mom’; you don't have to say ‘I love MY mom’. In Chinese, this applies to brothers and sisters as well.
I felt the same especially as duoling is teaching that at the beginning (and only learn here), but maybe it is logical, that it is your DeDe from the context and there is no need to stress it additionally ....
Because this sentence include "my younger sister and brother" it should be '我的'弟弟和妹妹。
With close family members it's often left out and also correct. (Mom, dad, brother, sister, etc)