No he was converted in the late 1930's as a student at University.
You seem rather obsessed with this notion of 'conversion' - so in your eyes, you
converted, your uncle
converted, this mysterious relative of yours
converted. So none were brought up as christians then? Or were they all (including you) brought up in a christian culture as christians, perhaps rebelled as teenagers and were comfortably back in the fold as adults (just like your idol CS Lewis).
If your family contains three people brought up in a non religious manner who actually
converted to christianity you'd represent a remarkable statistical quirk. Research indicates that just 1% of active christians in the UK were brought up in a non religious manner - for of the approx. 2 million active christians that would be just 20,000 people or one in one thousand of the approx. 12million people who say their upbringing was non religious.
So if this is a one in a thousand chance, three close members in one family all converting from a non religious upbringing to becoming christians represents perhaps a one in a billion likelihood.