You're right, Q...I was taking the mickey somewhat!
On a serious note, however, the one thing the questioner can absolutely count on - should he/she ever use the word 'aval' - is that virtually no-one will understand it, whereas everyone will understand 'grandparental'.
So obscure is 'aval' that even The Oxford English Dictionary - generally considered the 'bible' of the English lexis - does not even list the word in the grandparently sense! (Nor does it appear as such in the updated, online version of the dictionary.) It would appear, therefore, to be a very recent coinage despite its ancient linguistic heritage.
Yes, it appears in Chambers and on several websites dedicated to obscure words, but that's all that can be claimed for it.
But, as you say...what the hey! Cheers