Hi. My name is*****'m looking at your question now and will post my answer or ask for more information here in a short while.
The cost of the flat for CGT purposes is £90,000, its value when it was gifted to you. Therefore, as you sold it for £75,000, each of you and brother have made a capital loss of £7,500 so there can be no CGT. You can add the costs of selling the property (legal fees, selling agent fees etc) to the loss.
I hope this helps but let me know if you have any further questions.
I have to go out for a while but will be back to answer any follow up questions you may have.
It's worth reporting to HMRC so as to establish the capital loss you have each incurred. If you don't do that within four years of the end of the tax year in which you make the loss, you won't be able to use it against any taxable capital gains you may make in the future.