Any time there is a sudden change in behavior, it's best to take the kitty to the vet for a checkup. It's important to rule out anything medical that could be treated and solve the problem. If there's nothing wrong, then you move on.
You described Whiskey as an alpha cat! She's probably very territorial and overbearing on Frost. It sounds as if Frost has lost her confidence and is being bullied by Whiskey. You've had them for quite a while, so you'd think the behavior issues would have been ironed out by now. Each cat needs its own space and territory. That means cat trees, climbing shelves if you don't already have them. Besides, Frost needs a good escape route and bengals love to be high up in the air.
Feliway (plug in or spray) can possibly help. It's worth a try and can be purchased online or at a pet store. The other thing I might recommend is Jackson Galaxy's Spirit Essences. He has several holistic remedies for various behaviors and you may find one that will work for you.
http://www.spiritessences.com.
The thing to ask is whether there have been any changes in Whiskey's environment? Just the most subtle thing (like moving furniture around) could set this off. And now that you're pregnant, THIS could definitely be a problem. Cats are very sensitive to hormone changes in humans. Only Whiskey knows what's going on. I don't know that separating them and re-introducing them would help at this point. In episodes of My Cat From Hell, usually, it's cat-ifying your home (as I mentioned above). And distracting Whiskey when you think he's about to pounce on Frost. I know this means observing on your part, but if you truly want to fix the situation, this is most important.
Separate their feeding areas but feed them in the general same area (opposite ends of the kitchen, for example). You say you play with them. I'm guessing with both of them at the same time which keeps the distraction on the toys and not the other kitty.
The problem I see is that you have two small children and another one coming in just a couple of months. That alone makes your plate full. Add on the fiasco with the cats, and it's hard to keep sane. Enlist the help of your husband in dealing with the cats. Look around your home and make sure there are areas for them to climb, multiple sleeping areas for them. Make sure your children are safe from any possible attacks.
Just know there are some cats who do not want to be around children or noise. They want a quiet environment. This is not your fault -- it's just the preference of the cat. If you feel that things are simply not going to work out in the next couple of months, you may have to rehome one or both cats! They deserve to have a happy home and I don't know if finding a new home for Frost will help Whiskey or vice versa.
Your is not an uncommon problem, but it takes time and patience and fixing what's wrong and things will not change overnight. I hope I've offered you some suggestions to try. Good luck and let us know how things turn out.