A court in Germany rejected the application for an injunction, which aimed to impose a ban on Dr Stuart Gray and his brother Rory Gray talking to the press across Europe, including in the UK.
The court ruled the brothers can continue to call Dr Ubani a 'charlatan' and a 'killer' but could not call him an 'animal'.
Dr Gray said the ruling was a 'major milestone' for his family and that he would now concentrate on trying to have Dr Ubani struck off the medical register across Europe.
'It's a major step forward for us,' he said. 'If it was passed then we would not even be allowed to speak to the British press, which would have had fundamental ramifications for the freedom of speech.
'There is going to be a fitness-to-practise hearing through the local government, and we are pressing hard to get him struck off Europe-wide.'
Dr Gray said he was now considering how to efficiently distribute information about Dr Ubani, as he said the press in Germany is still prevented from talking about the case.
Dr Gray's father David died after Dr Ubani gave him an overdose of diamorphine on his first UK out-of-hours shift.