Don't worry too much, they're just babies yet. They'll only pick up your mood.
And they're Yorkies, not bigger terriers with real terrier temperaments like my troublesome pairs were. My yorkies have never been so gladiatorial
Feeding her by hand will be encouraging any dominant tendencies she has, she'll feel she's extra special in your eyes & a bit above Rhia, so you might want to think about stopping doing that.
This age is very important in training them to regard you as top dog, you really need to imprint that on them & there's lots of ways to do it kindly. You need to be firm but in no way shouty or aggressive, kindness is all in training. If you need specifics just ask.
It's essential with two litter mates to make sure you spend enough time with each of them on their own, one to one, instead of always having them as a pair. This is the very best way to stop them being so fixated on each other. Take them out for walks one at a time & do some training with them while you're out, every day.
If at all possible, find a puppy socialising class, either attached to an obedience training club or a ringcraft club for show dogs (doesn't matter if they're not show material, all puppies are welcome for socialising) to get them used to mixing with lots of other dogs & people. It all helps to dissipate that very intense bond between them that can turn to aggression if it's in their nature.
Hope this helps a bit.
xx
Meggie