All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
If your brother sold your share in the flat without your permission, then he should guarantee your share as he is considered a usurper by doing so. Whoever usurps something is obliged to return the same thing if he has the like of it, but if he does not have the like of it, he should pay its value on the day he took possession of it whatever its price was.
Therefore, your brother who sold your share without your permission is obliged to give you back a flat the same as the one he sold (half in size and equivalent in value) or give you its price at the time when he sold it. However, if your brother sold it with your permission and he kept its price with him as a debt, then you are only entitled to the amount which he sold the flat for.
Finally, we advise you to consider the circumstances of your brother and forgive him, as you mentioned that he sold the flat due to some unavoidable reasons, so you should not be too strict with him in getting your right back from him.