Half brother-in-law

Sir Raymond Bastable was referred to as the "half brother-in-law" of Lord Ickenham (Uncle Fred) in one of Sir Pelham Wodehouse's books. He was the half-brother of Uncle Fred's wife; that is the specified relationship. However, that unique and interesting term raises many other possibilities. Might he have been the full brother, but they were only half married? Maybe he was a full brother-in-law but only half grown? Maybe he was overly short in stature? Maybe the law was only halfway clear as to the relationship. All sorts of juicy ties suggest themselves. I wonder why Wodehouse did not pursue them.