"She has four older sisters."
We can't use the same logic in English. It is necessary to count like an adverb as in the example, or place the number after the noun, or use the particle の.
Some real life samples:
32 gentlemen and ladies joined our meeting today.
男１人と女３人で飲みますアドバイスください。 今度、俺（25）と女（25～30）で飲み会をします というより女子会に呼ばれたような感じ。
Please give me advice for 1 man and 3 women drinking. This time, I (25) and the girls (25~30) go drinking or feeling better called a girls' party.
Bodies of 5 young men and 1 young woman have been found in a passenger car. 3 persons at the front seats, 3 persons at the back seats died in a sitting posture.
25 women gathered on the basis of a single man compete at the aim of winning his heart.
Julestheman, 人 is the Person radical and can be pronounced nin, jin, and even hito, depending on how it is used. Most Kanji have two readings: the reading derived from the original Chinese kanji (On'yomi) and the original, indigenous Japanese readings (Kun'yomi). This site helps explain: https://www.tofugu.com/japanese/onyomi-kunyomi/
I'll try to explain as best as I can because it makes sense to me and i ran into this when i first started. It makes sense once you understand the grammer and how the particles work. Kanojo wa breaks up the meaning and identifies the subject while kanojo no joins them. Kanojo wa onessan ga - she is the subject, but her older sister is the topic to pay attention to or add emphaisis thats why ga is there. If it helps, just look at both her and her older sister as tbe subject with more emphasis being placed on the older sister. Kanojo no onessan - her older sister, both words are linked/l. If you said kanono no onessan wa yon nin imasu it would read like "her older sister" four of them has...WHO has?, but kanojo wa onessan ga yo nin imasu separates them so they aren't tied together if that makes sense. Specifically wa and ga separates them. Kanojo wa....STOP, ok we know the subject is her. Onessan ga....STOP ok now we know older sister is the topic we should pay attention to because of ga. Yo nin indicates four people and of course imasu is to have/has. Translated - she, older sister, four people, have. To make sense of these broken words once we look at it from a english standpoint and remembering the particles she has four people who are older sisters or simply she has four older sisters. Hopefully this helps someone.
In a normal text with context it can of course be omitted; For an isolated sentence here Duo would insist having that part explicit, and as there is no context provided the system would have no clue whether you have misunderstood. We can't do anything other than acknowledging it.
more about the ～人 counter here:
the kanji for 居る・いる and 有る・在る・ある are rarely used in text, so is not something you should learn to do, I think. This is especially true when using these verbs as auxiliaries.
There are some common words that use them though, like:
and the prefix 在～【ざい】which is used in words like:
在任【ざいにん】"being in office" as a politician for example.
How come when I answered 'he has x number of brothers' with the number before the brothers it was correct, but here the sentence structure requires that I put the number AFTER the sisters? It's the same type of sentence, but Duo wouldn't accept the same sentence structure??
The "correct" answer I was shown was: かのじょは四入のねえさんがいます。
The same sentence for "He has three older brothers" had the same answer without the "の" particle which I got "correct" (in actuality it's wrong; it requires the の if constructed in this way).
In both cases I wasn't given the option to use a "の" particle in the provided word blocks. Also, the honorific "お" is missing for the older sibling.
Unless I'm missing something and the “の” takes the place of the ”お” here?
Please first of all make the sentences consistent between brother sister, and secondly, provide all the word blocks to create the suggested correct answer.
PS. Or even better, show the answer that is actually wanted, and not an alternative that is impossible to write with the provided blocks.