can the fiancée visa be extended ?
Congratulations on your fiancée getting her FV ... great feeling isn't it!
Extended? Only in exceptional circumstances. It certainly should not be part of the plan. Sorry I am not following why her mother being at the wedding should cause a delay past the six month mark. The FV has been granted specifically so that the marriage can happen in the UK during the six-month period.
After she gets to the UK you need to wait about a fortnight before going to the Register Office to give the needed Notice of Intention to Marry. Given that she is a non-EEA Citizen that notice can only be given at a designated Register Office, that is, designated by the Home Office for immigration purposes and no doubt trained concerning checking the validity of visas etc..
Going to the Register Office does not necessarily bind you to have the wedding on a particular date although of course the Register Office can be booked at that time if you wish. The point is that until the Notice is given, it is posted publicly for about two weeks and then the necessary licence to marry is issued, it is not possible to get married. The licence is valid for 12 months, but of course you are up against the six-month validity of the FV.
