[matese]

You're on the right path, training with food reward and high verbal praise.

Whenever I would bring home a new nipping puppy my older girls would head for high grounds, the sofa.
It's normal for an older dog or one that has been the only fur baby to be scared of a new puppy.
Firstly a puppy to an older dog does not smell like a dog, so the older doesn't know what the heck is this thing momma bought home. It's normal for the older dog to stay away from crazy nipping puppy.
Big sister should have a safe haven, a place where she can get away from baby JAWS. It takes time for older dogs to accept a new kid in their domain, sometimes as long as two months where the older dog will stay in the same room with the puppy, but they will be BFF it just takes time.
As small as yorkies are, they play hard and rough.

Stay on that training program, sounds like Flint is getting it.
Spend no more than 5 to 7 minutes training sessions throughout the day as terriers have short concentration span. If Flint likes his food, you can use that as a reward treat also.
That photo is priceless lol
Thanks for sharing