Perhaps it's part your daughter feeling unsettled that makes it seem to her as though the babies aren't the same after time at his parents? I think for new moms there's a period of growth as a woman and mother where one begins to find their preferences or what have you.
I don't have twins. I did have 3 in under 4 years and by the time my third child entered my life I had become a little territorial if you will over well meaning advice.
At some point your daughter might want to experience motherhood on her terms not the terms of others, which is what I learned about myself.