However, it's been coming out in other ways. She's suddenly started being shy around her grandparents, with whom she has always been very close. When I ask her why, she says she's “lonely,” although that doesn't really seem to explain it. The way I interpret this is that she feels insecure, wants to emphasize her relationship with her mom and I, and feels that spending time with her grandparents will detract from that somehow.
She's been perfectly fine in the past when I had to be away from home overnight, even on occasions where I had to be away for three days at a stretch. Last night I went out for a few hours and she melted down, sobbing that she wanted Dada to come home. So, I can tell that sharing my free time with her baby sister is having an effect, even though she isn't blaming her sister for it.
I'm not sure what to do about all this yet. In the end it's probably just something that we all have to get used to. I just wish I had a magic wand to fix it all in an instant!