Non-related cousin?

Solution 1:

They are your cousins' cousins.

An English-speaking person wouldn't confuse this for a simpler relationship for which a single word or fixed phrase existed because (except where we want to avoid an ambiguity) we tend to select a formulaic word or phrase in preference to a less formulaic one for family relationships. In support of this, consider "brother" vs. "parents' son". "brother in law" vs. "wife's brother", etc. Thus if we see an unformulaic phrase like "cousins' cousin" we would guess it wasn't a circumlocution that referred to a cousin or sister or anything else that it could technically be.