ooc how normal is it to allow a child / younger sibling to win a game?

It's normal.

I figure that folks who play a competitive game with friends with any regularity either know they have to agree to some kind of appropriate handicap for the more advanced player(s) - or just don't realize that their regular match mates are doing it for them in some way less formal way.

Lucking into a perfectly matched opponent, who stays perfectly matched, happens, but is vanishingly rare.

An appropriate handicap is by far the best way to attract new players to a game. Modern games have built-in rules that explicitly acknowledge new-player mechanics, but older games tend to just have informal traditions for it.

Since I love to play games with folks, I often apply all of my skill to strategically coming in second place, with as close of a margin as possible.

People who notice me doing it don't tend to mind, but do usually try to punish me for it by destroying that narrow margin I'm trying to achieve. In the end, we still end up fully competing with all of each of our skills applied, which is great fun.