It's highly possible that your sister was cremated, not buried. In which case it may be worth ringing the crematorium nearest to the hospital or where your parents were living at the time. However, I doubt whether they will have records going back over 40 years. However, if you do come to a cul-de-sac, there is still no reason why you cannot ask your mother's local clergyman h to hold a small private service for your twin sister. It doesn't necessarily have to held on the site where she was buried or cremated if you can't discover this information and perhaps going through this ritual in a different way would help her. Clergymen are used to having what might seem unusual requests like this relating to bereavements and if you think it would help your mother where all else has failed, it might be worth pursuing.