On the other hand, it's also important that the two kids living in your house aren't treated as "your real daughter and the other one," and I don't think the letter really gives us enough info to say what kind of job the parents are doing at balancing those needs. They absolutely could be pressuring Abby--hi, we're your parents now, this is your sister, those are your grandparents!--but the LW only says that they told the grandparents that they had two daughters now, which seems like an understandable way to present it to people viewing Abby as a "guest". And if Abby has genuinely come to see the LW as her sister--which seems perfectly plausible to me--it would only be cruel to say "no, she's your godsister, but that's just as good!" It's a complicated situation.