I have no experience with the drinks you're talking about, so this is just for consideration. Over the years, I've read some about glycemic index. But I always thought with the typical meal of mixed foods that it would be difficult to account for. My understanding is that if you eat applesauce, your BS would raise faster than eating an apple of the same carb content. So, I'm wondering if the fact that these drinks are all liquid, results in a similar effect. If that were the case, taking the insulin in advance of consuming the drink might help flatten the curve. Of course, you have to be careful about where your mother's sugar is prior to this liquid meal. But even a modest lead time might help.
Another possibility might be to split the consumption of the Ensure so the effect of the sugar rise is spread out over time. If I was facing this situation, I would be taking multiple BS tests to determine what is happening to the curve. Of course, a CGM would be great, but I've done this with just test strips in the past. Hopefully, a few intense sessions like this would get you dialed in.
It's also important to note that if you are running high sugars, you are "peeing away calories" as Dr Bernstein has noted, regarding people unintentionally losing weight.
Hope that helps!