Gestational Diabetes Safe Cake??

As long as your sister limits herself to one piece, it won't matter what kind of cake it is. Moderation is the key, as they say.

I would recommend this cake, however you would have to make a different frosting because it uses cream cheese. You could substitute a non-dairy cream cheese if you can find it though. https://www.gnom-gnom.com/paleo-keto-chocolate-cake/
The best sugar alternative I've found is xylitol. Many sugar alternatives, natural or chemically made, have an aftertaste or leave a cooling sensation in your mouth. I haven't experienced that with xylitol. It is also the only alternative sweetener I've used that my kids will eat without asking if I did anything to the recipe.

What about flourless chocolate cake? I don't have a specific recipe in mind, but there plenty of good ones out there. Various recipes I've made don't usually have tons of sugar, more chocolate and eggs (sometimes almond flour) than anything else.

In addition to suggestions so far, consider:
* a not-cake celebration
1) fruit crisp or crumble
2) an indulgent vegetarian dish she likes)
* a not-on-the-date birthday cake & party right after baby is born
* Julia Child's gateau de crepes a la normande (made with apples, little sugar or a safe replacement, crepes which could be whole wheat or another complex grain)
* la low sugar or no-added-sugar crepe cake using whipped cream, dark chocolate, unsweetened cherries (flavors on my mind lately from talking about Black Forest Cake)
* something from a diabetic-purposed cookbook (at your local library, so you don't have to invest, or online)

I don't have a specific recipe- but there are plenty available if you search for cake recipes using artificial sweeteners. Since she's pregnant, I'd be more inclined to opt for using stevia instead, though. It's a natural sweetener derived from a plant, not made in a factory with chemical magic. That means it's more likely safe for the baby as well. And since she will also want to pay attention to eating more whole grains and complex carbs- consider making it with whole wheat or nut flours, or perhaps an apple or carrot version. Or maybe, consider a frozen yogurt cake? You could use non-fat yogurt, and sweeten it with fruit, possibly avoiding the need to use any artificial sweeteners at all. That gets her a sweet to enjoy, the dairy she will need, and fewer refined carbs to worry about. You can decorate that with printed rice paper decorations that can be ordered from various vendors at places like Etsy- or even perhaps your local bakery will help out. Just an idea to think about.

Have you considered cheesecake? Apparently it's ok if sugar-free and low carb. Here's one from a GB UK food site:

https://www.gestationaldiabetes.co.uk/recipe/chocolate-cheesecake/

https://www.gestationaldiabetes.co.uk/recipe/lemon-cheesecake-2/

I haven't tried either recipes, but they look really good.