Translation:My older brother will be in the hospital this weekend.
Hoping "My elder brother is in the hospital this weekend" gets accepted soon. No difference between older and elder
There "could" be a difference if your elder brother is into ancient majicks and lost lore. But I don't know whether the Chinese know about Lovecraft or not.
I used the similar answer "My elder brother is at the hospital this weekend." after previously using "older" for answers in this lesson and it was also rejected.
This is a bit strange since, if I recall correctly, the Japanese course allows the translation "elder" when describing siblings.
This sentence illustrates an interesting aspect of Chinese to English translation. 在 can be translated as "in" or as "at" as well as some other related meanings. The meaning of the sentence without any other context could be that his brother will be in the hospital for an operation or treatment or it could mean his brother is a medical professional working that weekend or even that his brother is a painter / carpenter / tradesman that will be doing his trade at the hospital over the weekend.
个 is a measure word, and its function in the sentence is to say the brother will be there for "this" weekend. 个 follows 这 and 那 to mean "this " or "that " respectively.
So 个 indicates future tence? How can you tell from this sentance that its in the future tence?
No, 个 is used with demonstrative adjectives (this, that) or numbers to point to a countable entity. 这个周末 means "this weekend".
Tense, on the other hand, is inferred from adverbs/adverbial phrases and other particles rather than direct modifications of the verb. In this particular case, the clue that the sentence is about an event going to happen in the near future is the adverb of time "this weekend".
Yes, 個 serves as a counter word here. But it is still OK if you say 這周末.
The 個 here is simply a counter word, but cannot say he only at the hospital for one weekend. If you say only at, you may say 我的哥哥只（在）這個周末在醫院, when the first 在 is optional. 只 means only.
can be : "is at the hospital" or "will be at the hospital" I suppose both are correct?
You can't without an explicit number of brothers in the sentence. It could be either. In a real conversation you would probably already know from context.
My older brother will be this weekend at the hospital
I do not understand why it is wrong.
It's too literal a translation and thus incorrect English. The time can't go in the middle like that.
"This weekend, my older brother will be in the hospital." or "My older brother will be in the hospital this weekend."
There is a big difference between "in the hospital" and "at the hospital". I don't know which one this sentence means.
Why can't, "My brother will be at the hospital this weekend" be accepted? I tried this and got it wrong even though it should have the same meaning!
Actually there IS a difference between "older" and "elder" and ge1ge is "elder", "older" is bad English.