"My younger sister is tall and also beautiful."
In this context "很漂亮" does not really mean "very beautiful" but rather just "beautiful". The only case where you would not use 很 in front of the adjective in a simple sentence like this is if you are implying a comparison. For example, if you say "我妹妹漂亮", you imply that your sister is beautiful compared to someone else you were just talking about.
If someone asks you "Why are we going by bus and not by car?" and you say "公共汽车快" (literally: bus fast), it means "the bus is faster than the car". If you reply "公共汽车很快" (literally: bus very fast), it just means "the bus is fast" without commenting on which option is faster.
So because I've used "我哥哥" before and it didn't work, I knew that writing the sentence as “我的妹妹又高又漂亮” and as I expected I got it wrong. I believe that if a website/app has a goal to teach people other languages, it should include things like grammar, or even culture (Ex. in China they say 喝汤=drink soup, in stead of eat soup=吃汤). I am actually not a native Mandarin speaker, but I am using Duolingo, to help me continue to practice Chinese, and learn more. But it is difficult because I have to adjust from the way 我的老师 had taught me. She was born and raised in China, and taught us about different grammar usage such as: “虽然。。。但是／可是” “非。。。不可” “除了。。。以外” “又adj/verb又adj/verb” “因为。。。所以” Hopefully, all of us who desire to learn Chinese will be able to learn more grammar in the future.