Since she was born after 1923 she should be in the danish CPR-register. If you can prove that you are related there might be some help.
Try to contact the authorities in Copenhagen at this email adress:
Give the name you are searching for : Aase Nielsen
Birthdate 30 june 1925
and the info that she emigrated to America and now is dead.
Then see if they can help you.They might charge a fee.
Hope this helps.