It may sound strange, but I think part of their lack of interest is caused by DD looking so much like my husband's side of the family. Well, judging by the emails we've recieved where they've asked us to please have another child that looks like them. Okay then, crazy parents! I'll get onto spitting out a clone for you!
NOS, you need to grieve for your sister. No one can change who she was and no one can grieve for you. It was selfless of you to help your parents with it but you deserve support too. The loss was as much yours as theirs. I'm so glad you have your letters.