I realise that this isn't the main point of the letter but I am not sure the parents are handling this quite right. I don't think a 12 year old who has lost their parents needs "new parents" and a sister and new grandparents, I don't think it's possible to just try and slot her in or even desirable for her. I think they would be better off presenting it as they are godparents in loco patentis, the OP is a godsister not sister sister. I don't know if I am explaining this well but I think the parents are basically trying to simplify a relationship - she is now a daughter - that cannot ever be that simple.