That's a question I'd like to find answers to as well! My son (age 20) was Dx'd with Type 1 at age 14, and for the first few years he WOULD wake up at night when he was low. Over the past couple years though he DOES NOT wake up and had one severe seizure earlier this year with a bg of 28. We live alone and I work nights until 3 or 4AM. He was setting his alarm for 2AM most nights, but usually he would just turn it off and go right back to sleep. I always check his bg when I get home, but the night of his seizure I was late, and apparently his BG had been very low for too long a time. This is my biggest fear now, and I worry about him going low at night all the time, so I started calling him on the phone at 2AM instead of relying on his alarm clock. Some nights I call every 2 hours from the time he goes to bed until I get home. I know this seems excessive, but what are my options? His seizure was VERY frightening (he stopped breathing and was unconscious), extremely painful and detrimental to him, and I don't ever want him to go through that experience again. A CGMS (Continuous Glucose Monitoring System) would be great to have, but Medicaid does not cover it and it's very expensive. I sometimes wonder if he'll ever be able to live on his own, and especially alone. It seems unlikely.
Whenever I have a low during the night, my body usually wakes me up itself. But I guess it's not the case with everyone. Maybe your sister could set an alarm for say around 2 or 3 in the morning so she can test her blood? And always having snacks and juice on hand by the bed is really good.
Hope that helped some!