It could be an idea that your brother informs them at check-in or even a couple of days before that you will not be using the outbound flight from UK.
The alternative is that you will be marked as a "no show" which sometimes can lead to the tour operator cancelling the return flight leg and possibly your transfers in resort.

I think it's better to come clean so to speak and make them aware of your plans....

As Cyprus says definitely inform them or you may loose accommodation.

The TO won't care as they can sell your flight.

If you don't use the outbound flight, the inbound flight (although I'm not sure you're using that either?) transfers, and unless you're sharing a room or apartment, your accommodation are likely to be cancelled. You do need to get it authorised before you just miss an outbound that's part of a package.