"Your father-in-law is big."
Translation:Mae eich tad yng nghyfraith yn fawr.
Presuming you remembered to mutate "Tad" to "Dad" after "Dy" is should have been accepted.
You've slightly misspelt nghyfraith there, but it probably didn't accept it because you need the soft mutation after dy: dy dad-yng-nghyfraith.
In English, there is a strict rule about the hyphens in "father-in-law". Does this also apply in Welsh? Thank you.