Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear about this new conflict between Snowflake and Sunshine.
When animals suddenly change their attitudes toward one another it means that something has changed. Either there is some sort of stress in their lives, resources (including time with you) are less or someone isn't feeling well. Dogs are very keen at picking up a change in body odor or a change in the way a housemate is moving which can signify sickness.
When an animal doesn't feel well they are less likely to tolerate housemates. And it is natural for healthy animals to attack and drive away a sick animal away from their territory. Although it seems cruel it is an adaptive response which is quite natural.
Also a once submissive housemate may take this opportunity to take advantage of a once dominant housemate. If the unwell, former dominant one, fights back then the war is on until some sort of understanding is reached.
So ideally both should have a thorough physical examination, urinalysis and bloodwork to make sure all is OK. Make sure their urine is checked for any signs of crystals or infection and cultured to make sure a subclinical low grade infection isn't part of the problem. Bloodwork to look for internal organ disease and hypothyroidism is advisable too.
I would keep your two separated. We need to give them both some peace while we get this sorted out, and the more fighting that is allowed to happen the greater reinforcement for a shift in social status and confidence that it is ok for Sunshine to attack Snowflake.
I would use the same brush on both so that their smells are familiar to one another. Rotate beds and toys between the two areas so that nothing becomes one dog's only.
If they seem to settle a little perhaps you can try limited visitation after they have had several days to calm down, but only when you are home and can supervise to change or alter behavior. I would start with both on a leash so they can be quickly separated if needed. Make sure to have plenty of resting places and toys so each can have their own place to be comfortable and things to play with so there isn't competition for resources.
If either dog starts to behave aggressively use an airhorn to startle her. An immediate negative consequence that is always present and not physical may be enough to help change behavior.
In some cases oral medication is needed as well such as fluoxetine or amitriptyline as calming agents to decrease the stress and aggression. These don't need to be forever, just until we can settle them down and they learn to live in peace again, or Snowflake is well again.
I do find that when dogs that previously got along fine are now fighting something changed, that either medically or socially there's been a shift. It can be difficult to figure out and fix but that's usually the only way to restore peace.
Best of luck with your pups. Let me know if you have any further questions.