Sty and style are not related;
neither are braid and bread…
But some words like river and rival
surprisingly are, and more obviously,
void and avoid.
The bad news is that river and rival aren’t actually related, since the Latin word ripa ‘(river) bank,’ the root of river, is of unknown etymology; the good news is that rival has a very interesting etymology which is the real focus here: it’s from Latin rivalis ‘one using the same stream as another, a rival,’ from rivus ‘stream.’ Isn’t that unexpected and delightful?
And while we’re on the subject of interesting etymologies, check out the tangled history of apricot:
Alteration of earlier abrecock, ultimately from Arabic al-barquq, the plum : al-, the + barquq, plum (from Greek praikokion, apricot, from Latin praecoquus, ripe early : prae-, pre– + coquere, to cook, ripen; see pekw- in Indo-European roots).
I will add the last sentence of the relevant entry in Waverley Root’s endlessly readable and highly recommended book Food: “The chief consumers of apricots after man are dormice, who dote on them.”