Welcome, tedolita, and thanks for sending details to the Appeals Box.
In-year appeals aren't really any different from appeals for the normal point of entry. We've certainly had successful in-year cases on the forum in previous years. The school may be heavily oversubscribed, but that could apply to the normal point of entry too. The criterion is quite simply whether the parents' reasons for wanting a place outweigh any prejudice to the school if they have to take on an additional pupil.
By all means mention the difficulties you've encountered - but keep it very brief because it's not going to determine the outcome of the appeal. The most you can hope for, if you're brief and to the point, is to elicit a bit of sympathy from the panel.
By all means mention that a sibling will "almost definitely" be going to the school in the future - but I doubt that any panel will give much weight to something that has yet to happen.
I do like the fact that you say the allocated school is "great" because that will go down well! Mention briefly why this school is not appropriate, but your focus really needs to be on the school you're appealing for. (I'm assuming the only issue at appeal is going to be oversubscription.)
Look carefully at section C of our Q&As, which deals with oversubscription, and at the sort of reasons you might be able to put forward:http://www.elevenplusexams.co.uk/appeal ... -school#c2