Translation:Eich tad yng-nghyfraith
See the discussion above. There is usually no hyphen in yng nghyfraith. This was a typo in the first version of the course which we are unable to correct. (Once a course is ‘locked’ for release, even as a beta, no new words or phrases can be added to its database by the course teams.) It is only a minor thing, though.
I thought the usual form was with. Gweiadur (s.v. cyfraith) gives the "phrase" yng-nghyraith with an example of brawd-yng-nghyfraith.
On the other hand, if you look up brawd, it has an example phrase of brawd yng nghyfraith.... Ditto for tad, tad yng nghyfraith; mam, mam yng nghyfraith; chwaer, chwaer yng nghyfraith.
How inconsistent within the same dictionary!