Thats the thing, its been YEARS since she died and buried, so only skeleton remains (as stated), the moment the curse break, its just bird skeleton morph into human, if they did a shallow grave, at the very least you'll see a small bulge increase, if its deep, then it wont change much at all until you dig it
I feel bad for the mother tho, it seems she realize at one point its wrong to obtain love via sorcery and stopped doing it, (thats when the husband stick around at house less and keep working abroad more, and she sees the reality of everything), she also seems to not be one of those typical trope of evil and horrible step mother, she seems to treat our FMC very well and tried to be good mother to her
and at the end she feel its so wrong to leech everything so far and tried to convince her daughter, her flesh and blood to abandon this family and return to their home country, only to be betrayed by her and get cursed, she tried so hard to make people notice her, I guess thats when the accidental bird cage fall happen
The thing is I dont think bird will die if theyre properly in cage , at the very least just injured, unless theyre purposely drop at a very high place on purpose and that makes methinks the evil villain did it but convince herself over and over its an accident