I believe so. They have 4 different words for older sister, younger sister, older brother and younger brother

yeah

I did sister and i still got it correct. But it also said another transaction and it said i could have used older sister.

是 is specifically used for equality, eg "She is my older sister", not for descriptive "is" sentences (noun is adjective). In that case, you need another word, usually 很 if no other adverb-ish word is appropriate.

As an example of another word to use, 她很忙 could be negated as 她不忙. I believe 她最忙 would be the correct way to say that she is the busiest as another example.

Because in Chinese they don't use "Is" for something that already is something. For example, You don't say " She is very pretty" If she's already to be pretty! instead, you say Tā hěn piàoliang 她很漂亮 literal translation means "She very pretty". I know for us that makes no sense, but that's just how Chinese works.

So to sum up, noun + is + noun = noun shi noun, noun + is + adjective = noun hao adjective.

There isn't a plural form of the term. You'd just say the number of older or younger sisters you have followed by the classifier for the number of people followed by the older or younger sister term. This is the same with all nouns in Chinese. You'd just say the number of things followed by the classifier for that thing followed by the term for that thing.

men 们 is mostly just for pronouns.