The general rule is f changes to ves as words become plural

The general rule stems from the linguistic fact that, in Old English, the f (along with a couple of other unvoiced consonants) became voiced when between two consonants. Thus OE wif, pronounced [weef] and meaning woman was wifes, pronounced [wee-ves] in the plural. Thus Modern English wife, wives. This rule can slop over onto words brought into the language when the force of the rule is still what the linguists call productive.