What's wrong with: "La famille est sacrée !"
It is accepted now but Duo says I have a typo.
It would seem "c'est sacre" is set expression and doesn't modify the noun. I found several examples on the net.
Are you sure? Is it an idiom, then?
Why doesn't 'sacre' agree with famille and have an extra 'e'? The answer does not offer one. But surely it should be 'la famille est sacrée' or is that reserved for the holy family?
I think Duolingo should stick to its own teaching protocol and put the second 'matching' (feminine) 'e' on the end, whether or not it is an idiom without it...