OK, thanks for that. It is important. In order to get married in the UK your brother will need to apply for a Certificate of Approval to Marry ("CoA") and had he less than 3 months validity left on his visa that would not have been possible.
Click here to download a CoA Application form. That application should be made as soon as possible. It will take about two months to process.
Following the issue of the CoA your brother and his fiancée need to go to a designated Register Office and register their intention to marry. That will result in permission to marry being issued after about two weeks, and that permission is valid for 12 months. However in your brother's circumstances that marriage clearly needs to happen before the expiry of his current visa.
If the two of them intend to get married in a (specifically) Church of England Church then post that fact and ignore the above.
