For many years I had a brick wall in my family history research.
I could find information about my great x3 grandfather, George Mackillop (1790-1860) in a variety of sources including documents. Often the name, James Mackillop, also appeared in the same documents. I was certain that they were brothers but I needed proof. I also wanted to know the names of George's parents.
One day I decided to do some lateral research and instead of searching for information about George I decided to research James. I knew that, like George, he was a merchant in India in the 1800s so I decided to search British Newspaper Archive available online via some library websites for any references to James. Apart from references to where he lived and that he had made a great deal of money as a merchant I found articles mentioning that he was, at one time, a member of parliament.
A search in Google for "James Mackillop" member of parliament took me to the following post on the History of Parliament website. I had hit the jackpot.
The first information provided was that James Mackillop was the first son of John Mackillop and Mary who was the daughter of Robert Downie of Kilmadock, Perth. James was unmarried and died on 27 January 1870. In the following biography it was stated that George Mackillop was almost certainly his younger brother and that they both worked in India for a time in the firm where their uncle, also Robert Downie, was a partner. I knew from other sources about their association with the Downie family but it was only when I discovered their mother's connection to that family that it all made sense. Both James and George had travelled to India when young to work, initially, in the family business. They then made their own way as merchants.
This is an example of the benefits of researching other family members, not just the direct family line, and also looking at a wide range of sources that may provide a missing clue.