I'm worried about my younger sisters strange behaviour? Please help?

A few months ago in January, my dog was put down as he was very old and weak. He had been part of my life ever since I was 4 and a part of my sisters life since birth. He was loved so much and we saw him as more part of the family than a pet. I'm now nearly 18 in June and she will be turning 12 in September, however she has created an account on Facebook as though it's him. I saw a little earlier how she has even been talking to him on it through private mail, about every day things like boys and school and although it's her writing on both accounts - she's made it look like he's been giving her advice about everything and even in one message he's said he loves her more than he loved me or my brother - which is something she always said to me when he was alive. It never bothered me because she's only young and I knew it wasn't true anyway, but it now seems like she's become infatuated with him and she needs to hear him say he loves her more. We all struggled with his death, however as it was months ago, I thought we were all, not over it but.. I thought we had all came to terms with it. She begged me not to tell my dad and started crying saying she hates me for seeing it, when I never tried to read her mails... I Still get upset when I see pictures etc but is this her way of dealing with the loss? Or should I really say something?