Prince Philip and his sisters and their families were very close, and he made a point of travelling to Germany (usually without the Queen, who was always too busy) whenever there was a wedding or some other major family event. At one time when his widowed sister Sophie was to marry again, he borrowed a Canadian army vehicle and dashed across war-ravaged Europe to turn up unexpectedly in time for the wedding in Salem in May 1946. Until her death in 2001, Sophie ("Tiny") was a frequent visitor to her brother and sister-in-law – as her children and grandchildren still are.
Princess Margarita, Philip's eldest sister, was Princess Anne's godmother. Princess Sophie, his youngest sister, and Ludwig von Hessen und bei Rhein, brother-in-law of Prince Philip's sister Cecilia and a first cousin once removed of Prince Philip, were two of Prince Edward's godparents.
Prince Philip's niece Princess Friederike of Hanover is the Queen's goddaughter, and Philipp, Prince of Hohenlohe-Langenburg, Princess Margarita's grandson, is Princess Anne's godson.
Prince Philip, who is fluent in German, also used to be a frequent visitor at the hunting lodge Schloss Wolfsgarten to see his Hesse and Battenberg relatives. There they have a windowpane doubling as a guestbook with scratched signatures and dates. The name with the most dates by far (about 3 dozen) is 'Philip' but you also find 'Nicky' and 'Alix' and 'Anne' and 'Mark' and 'Charles' and 'Diana' and a very special one: 'Lilibet 20 May1965'.
On their last state visit to Germany, the Queen and Prince Philip had lunch with at least two of their German relatives: Prince Donatus, Landgrave of Hesse, who is the successor of Prince Philip's short-lived brother-in-law Prince Georg Donatus, and Maximillian, Margrave of Baden, the son of Prince Philip's sister Theodora.
On the day of his platinum wedding anniversary, Prince Philip was pictured carriage riding in the grounds of Windsor Castle alongside his nephews Max and Ludwig of Baden.
Donatus of Hesse, not a nephew of Prince Philip but a descendant of Queen Victoria via her eldest daughter, is a horse breeder like the Queen and has accompanied her to various events, like the Royal Windsor Horse Show. He gave her a 5-yo. premium Trakehner mare called Dolce Luciana as a Diamond Jubilee anniversary gift.
Charles and his siblings certainly know all their German cousins very well.
Below: Prince Philip at the 1957 wedding of his niece Margarita in Salem, West Germany. He also attended her funeral in 2013.