I told them that, while I loved them, I loved Betty best. She was my favorite child. Never made noise, never caused trouble. Betty was also invisible, and, in fact, did not exist. They seemed to dump all their jealousy on her.

My opinion is that there should be less discipline during this pandemic. The kids are cooped up more, probably also feeling the frustration from the parents. Unless they are actively hurting each other or breaking things, I would not yell or discipline.
Ten years from now, you’ll realize that your 5 year old was really still a baby.

1. Do not put one sibling in charge of the other, it might be convenient to you in the moment, but longterm is damaging to the sibling relationship.

2. Avoid comparisons.

3. Insist on groundrules for example don't allow name calling

4. encourage, toys, games, and activities that will have them working together.

5. Try not to over intervine, as long as they stay in the guidlines for respect let them work out their squabbles.

6. Set some time aside each day where you spend some time with them, no distractions from your phone or Tv. you go for a walk together, play hide and seek in the house, something where they are having, fun but don't have to compete with you for attention.

So I am going to go against the crowd, and say something that isn't going to be popular, and people will probably flame me for and say I am projecting.
How you described your daughter's words and behavior. Very much reminds me od my sister and her behavior towards me. This is how she acted our entire childhood.

The individual attention, and we're a team aspect was not enough for my sister. I do not want to diagnos your daughter, but please consider therapy if you do not see improvements with the things being suggested in this thread.

Keep a close eye, my sister was very good at appearing to be playing kindly, but being very cruel..

Unfirtunately things like this wren't taken quite so seriously in the late 80s and 90s, so my sister has grown to be quite the terror in the family, and I have had to establish very firm boundaries. I think if things had been reconized earlier, and she had been able to receive helo beyond the advice of more attention, she could be in a better place and we could have a better relationship.

Again, I don't want diagnose your daughter. Just don't assume she'll grow out of it, and be open to her neededing more help that just one on one activities.