Translation:I love my younger brother and younger sister.
Should it also consider the plural, like in
I love yonger brothers and sisters?
You would need to say "I love MY younger brothers and sisters" and this should be accepted by Duo as correct. Your sentence (without the possessive adjective MY) is in the format of a generalisation, which makes no sense when referring to your brothers and sisters and is therefore wrong. However it would be OK for something like "I like cats and dogs" because this refers to all cats and dogs.
There would be a measure word like "dou" (all) to refer to more than one. If you love a specific # of them you use a number followed by the appropriate measure word "kou" (translates as mouth to feed) although they use "ge" on here. "Ge" is a universal measure word (when in doubt). There is one for flat things like beds and paper, or long things traditionally made from bamboo (like pens). Just google measure words to get the concept.
It is important to acknowledge that this brother and sister are both specifically younger than the speaker. Knowing whether the people being talked about are younger or older is VERY culturally relevant and important to remember, because when meeting people who speak chinese, people often refer to each other as 姐姐、妹妹、弟弟、哥哥、etc. based on how old the person they are speaking to is, even an aquaintance. You would also call people a generation older than you 阿姨 or 叔叔 and based on whether you know that person through your mother or your father, it can change even more. But for now this is all you really need to know so just practice getting these terms right and you'll be fine.
Clarification: without "我的", "弟弟" and "妹妹" become proper nouns. It's actually the same in English; in "I love my mom and dad", the words "mom" and "dad" are common nouns, but in "I love Mom and Dad" they are proper nouns. We see English native speakers (including the designers of this course) writing "I love mom and dad" without capitalization, but this is out of laziness or ignorance; it doesn't change the fact that the words are being used as proper nouns here.
So there is a problem with this question. "Brother" and "Sister" are not commonly used as proper nouns in English the way their counterparts are in Chinese.
Proper nouns can be used as forms of address, and they generally lack the countable/uncountable feature that common nouns have.