Thanks all for the suggestions.
The one item that came up repeatedly from you all and the educators was “Would you leave her alone if she did not have T1D?” The answer to that question is, “I’m not sure I would.” Her older brother should not be designated caregiver just because he was born first. This has been a theme their entire lives. Everyone who is an eldest child knows exactly what I am talking about.
Then, depending upon that answer, the next question is how to mitigate the risk since she is T1D. My child has been exceptionally angry lately. (On top of diabetes, her dad and I divorced.) The outcome has been trouble following basic self-management: testing enough during the day, testing before eating, checking for ketones, and worst of all, lying to me and her about testing.
With all that mind, we opted not to allow her to stay at her dad’s if he was not going to be there (regardless of where her brother was).