Both Ernest Augustus I of Hanover and George V of Hanover (his son) were blind. George became blind as a teenager, and his father in his later adulthood.
Augustus Frederick, son of Prince Augustus, Duke of Sussex, who himself was the son of George III of the UK, very likely suffered from multiple sclerosis. The disease was not identified during his lifetime, but symptoms he recorded in his diary over many years are very consistent with the disease.
Although not German princes, Prince Erik of Sweden (1889-1918) and Prince John of the UK (1905-1919) both had disabilities. Both Princes were epileptic; Erik was mentally disabled and John likely had Asperger's Syndrome.
And way back in the 13th century, Henry III of England had a deaf daughter, Katherine, who died at the age of three.